1) You can get custom scenery in the game by just opening parks that contain custom scenery. It doesn't automatically add everything to every savegame of course, because the objects available are selected per savegame. OpenRCT allows you to easily access the object selection window while playing though, unlike normal RCT where you would have to use an external tool like ParkDat, or you'd have to load your park in the scenario editor. Just download shit!
2) In the object editor, you can right click and paste. Gotta copy something first of course. If your selection is wider than the object (a single tile is 64 pixels, I believe), it will get resized. Also note that you're stuck with 256 colours, so images with full colours will appear differently. Play with it.
Oh, a bug in the object editor: in one of the angles, the colour of the top left pixel of your selection will be made invisible for the entire image. May want to manually put a weird pixel in the top left, and edit that out after you've pasted it.