The GUI contains all the hard coded images of RollerCoaster Tycoon 2. These images are saved in the g1.dat, which can be opened with the so called RCT2GUIEditor. The following list shows under which number you can find the pictures you are looking for.
Land Farthest Zoom
1-37 - Grass
38-56 - Grass (Underground)
57-94 - Grey Sand
95-113 - Grey Sand (Underground)
114-151 - Red Sand
152-170 - Red Sand (Underground)
171-208 - Yellow Sand
209-227 - Yellow Sand (Underground)
228-265 - Ice
266-284 - Ice (Underground)
285-322 - Dirt Grass
323-341 - Dirt Grass (Underground)
342-379 - Abstracts
380-398 - Abstracts (Underground)
399-436 - Martian
437-455 - Martian (Underground)
456-493 - Checker 1
494-512 - Checker 1 (Underground)
513-550 - Checker 2
551-569 - Checker 2 (Underground)
570-607 - Dirt
608-626 - Dirt (Underground)
627-664 - Grey Rock
665-683 - Grey Rock (Underground)
684 - Ride Editor
685-703 - Owned Land
704-722 - Construction Rights Owned
723-741 - Land Tool (De-Selected)
742-760 - Land Tool (Selected)
761-765 - BLACK
Land Middle Zoom
766-803 - Grass
804-822 - Grass (Underground)
823-860 - Grey Sand
861-879 - Grey Sand (Underground)
880-917 - Red Sand
918-936 - Red Sand (Underground)
937-974 - Yellow Sand
975-993 - Yellow Sand (Underground)
994-1031 - Ice
1032-1050 - Ice (Underground)
1051-1088 - Dirt Grass
1089-1107 - Dirt Grass (Underground)
1108-1145 - Abstracts
1146-1164 - Abstracts (Underground)
1165-1202 - Martian
1203-1221 - Martian (Underground)
1222-1259 - Checker 1
1260-1278 - Checker 1 (Underground)
1279-1316 - Checker 2
1317-1335 - Checker 2 (Underground)
1336-1373 - Dirt
1374-1392 - Dirt (Underground)
1393-1430 - Grey Rock
1431-1449 - Grey Rock (Underground)
1450 - Ride Editor
1451-1469 - Owned Land
1470-1488 - Construction Rights Owned
1489-1507 - Land Tool (De-Selected)
1508-1526 - Land Tool (Selected)
1527-1575 - BLACK
Land Edges
1575-1662 - Dirt Land Edge
1663-1746 - Black Mine Edge
1747-1830 - Brown Mine Edge
1831-1914 - Ice Land Edge
Land Closest Zoom
1915-1952 - Cut Grass
1953-1971 - Cut Grass (Underground)
1972-2009 - Grey Sand
2010-2028 - Grey Sand (Underground)
2029-2066 - Red Sand
2067-2085 - Red Sand (Underground)
2086-2123 - Yellow Sand
2124-2142 - Yellow Sand (Underground)
2143-2180 - Ice
2181-2199 - Ice (Underground)
2200-2237 - Dirt Grass
2238-2256 - Dirt Grass (Underground)
2257-2294 - Abstracts
2295-2313 - Abstracts (Underground)
2314-2351 - Martian
2352-2370 - Martian (Underground)
2371-2408 - Checker 1
2409-2427 - Checker 1 (Underground)
2428-2465 - Checker 2
2466-2484 - Checker 2 (Underground)
2485-2522 - Dirt
2523-2541 - Dirt (Underground)
2542-2579 - Grey Rock
2580-2598 - Grey Rock (Underground)
2599-2622 - Staff Working Area
2623 - Ride Editor
2624 - Ride Editor (Underground)
2625-2643 - Owned Land
2644-2662 - Construction Rights Owned
2663-2738 - Mown Grass
2739-3042 - Uncut Grass
3043-3061 - Land Tool (De-Selected)
3062-3080 - Land Tool (Selected)
3081-3099 - Quarter Tile Placement Grid
Construction
3100-3110 - Colour Palette
3111-3114 - Direction of Placement Arrow
3115-3118 - Maze Construction
3119-3122 - Direction of Placement Arrow (Diagonal)
3123 - Map Edge
3124-3389 - Steel Supports
3390-3394 - Wood Supports
3395-3464 - Pathing
3465-3657 - More Wood Supports
3658-3704 - Mine Train Supports
3705-3708 - Gentle Slope Wood Support
3709-3716 - BLACK
3717-3860 - More Mine Train Supports
3861-4914 - BLACK
4915-5047 - More Colour Palette
5048-5052 - BLACK
5053-5057 - Water Effect
5058-5640 - Interface
5641-6408 - Height Markers
Peeps/Staff
6409-11260 - Peeps
11261-1140 - Handyman
1144 -11880 - Mechanic
11880-11972 - Security Guard
11972-12128 - Panda
12128-12324 - Tiger
12324-12544 - Elephant
12544-12676 - Gorilla
12676-12892 - Snowman
12892-13104 - Knight
13104-13376 - Bandit
13376-13608 - Pirate
13608-13764 - Sherrif
13764-13896 - Astronaut
13896-14184 - Roman
14185-14221 - BLACK
Rides/Other
14222 - 14403 - Preview image ??
14404-14556 - Mini Golf
14557-14563 - Roto-Drop
14564-14566 - Launched Freefall
14567-14571 - Station (Tower Rides)
14572-14985 - Bobsled Coaster
14986-14988 - Observation Tower
14989-14993 - Station (Observation Tower)
14994-15003 - Elevator
15004-15805 - LIM/Looping Track
15806-16223 - 4D Track
16224-16819 - Corkscrew/Hyper Track
16820-16869 - Cycle Monorail Track
16870-16899 - Helicopter Ride Track
16900-17025 - Spinning Wild Mouse/Steel Wild Mouse
17026-17145 - Hairpin Rollercoaster
17146-18073 - B&M Track
18074-18738 - Giga Track
18739-19337 - Mini/Spiral Track
19338-19661 - Mine Ride (Powered) Track
19662-19719 - Impulse Track
19720-20051 - Waterslide Track
20052-20499 - Mine Train Track
20500-20539 - Chairlift Track
20540-20547 - Chairlift Station
20548-20567 - Slide
20568-20751 - Slide (Peeps)
20752-20819 - Go Karts
20820-20995 - Splash Boats Track
20996-21131 - Log Flume Track
21132-21293 - River Rapids Track
21294-21457 - Heartline Track
21458-21503 - Virginia Reel Track
21504-21605 - Reverser Coaster Track
21606-21919 - Side Friction Track
21920-21937 - Space Saucers and Dodgems
21938-21993 - Mazes
21994-21997 - Pirate Ship Supports
21998-22001 - Inverted Pirate Ship Support Beam
22002-22133 - Magic Carpet
22134-22149 - Ride Bases
22150-22153 - Ferris Wheel Supports
22154-22225 - Reverse Whoa Belly Track
22226-22361 - Air Powered Track
22362-22391 - Ground Station Parts
22392-22403 - Inverted Station Parts
22404-22411 - Water Stations
22414-22417 - More Ground Station Parts
22418-22425 - More Inverted Station Parts
22426-22432 - Ride/Station Bases
22433-22444 - Wooden Station Cover
22445-22456 - Canvas Station Cover
22457-22468 - Grey Castle Station Cover
22469-22480 - Brown Station Cover
22481-22492 - Jungle Station Cover
22493-22504 - Log Cabin Station Cover
22505-22516 - Roman/Classical Station Cover
22517-22540 - Abstract Station Cover
22541-22552 - Snow Station Cover
22553-22564 - Oriental Station Cover
22565-22576 - Space Station Cover
22577-22636 - Shards and nails and other crap stuff in coaster crashes
22637-22650 - Train Smoke
22651-22658 - Escaped Balloon
22659-22663 - Smoke???
22664-22679 - Plain Entrance/Exit
22680-22695 - Canvas Entrance/Exit
22696-22711 - Classical/Roman Entrance/Exit
22712-22727 - Brown Castle Entrance/Exit
22728-22743 - Grey Castle Entrance/Exit
22744-22759 - Log Cabin Entrance/Exit
22760-22775 - Wooden Entrance/Exit
22776-22791 - Jungle Entrance/Exit
22792-22823 - Abstract Entrance/Exit
22824-22839 - Snow Entrance/Exit
22840-22855 - Oriental Entrance/Exit
22856-22871 - Space Entrance/Exit
22872-22877 - Border Fence
22878-22926 - Explosions
22927-22954 - Crash Splashes
22955-22956 - For Sale Signs
22957-22972 - Smoke..
22973-23036 - Jumping Fountains
23037-23100 - Rain
23101-23132 - Litter
23133-23188 - Ducks
23189-23198 - Weather Icons
23199 - Scenario Completed Tick
23200-23206 - BLACK
23207-23230 - Menu and Logos
2323-23340 - Monorail Track
23341-23452 - Train/Tram Track
23453-23484 - Wooden Coasters Loop
23485-23496 - Camera
23497-25228 - Wooden Coaster Track
25229-25852 - Stand-Up Coaster Track
25853-25960 - Suspended Monorail Track
25961-26226 - 4D Inverted Track
26555-27128 - Vekoma Inverted
27129-27806 - B&M Inverted
27807-28382 - Junior Coaster/Watercoaster Track
28383-28522 - Single Rail Suspended
28523-28534 - Boat Ride Track
28535-28632 - Wooden Wild Mouse Track
28633-28772 - Steeplechase Track
28773-28820 - Car Ride Track
28821-28882 - Ghost Train Track
28883-28958 - BLACK
28959-29012 - Land Textures...
29013-29109 - Water Splashes
29110-29292 - rotating key