laladk, I don't know why you can't download the instruments, but about the instrument pictures:
These are just "icons" that you can attach to tracks in GarageBand. They aren't real (software or otherwise) insturments and you can actually put an icon for drums, for instance, on a keyboard track and the sound will not change. The icons are just eye candy (iCandy, if you'll pardon the pun).
If you are still under the 90 days, or if you bought Apple care, I'd call them.