I know Blitz3D has a poly limit
No, it doesn't have a limit. Every poly counts, of course, but it doesn't have a limit.
What I mean is that if you were to load the game with tons and tons of pictures, would it slow down?
Of course. Drawing them takes cpu/gpu time and storing them takes memory/videomemory. Loading more things into videomemory than you have vidram for can be particularly nasty. You don't want to be getting all anal about everything and end up cutting things out of your game though. Just be aware that it all adds up, and make sure that everything you load and everything you draw *needs* to be there, is adding something to the game, and that you couldn't do it better another way.