Originally I wanted to have actual sprites for the big stuff instead of just resizing them (which I did do for coins and bullets), but that wasn't a priority. If I ever update the game, I'll probably end up doing that.
As for movement, I'm not really sure what's slippery about it. It's clearly an issue since others have brought it up too, but it feels fine to me. Like you can stop exactly where you want to, which you wouldn't be able to do if it was slippery, so I'm not sure. What specific thing about it feels slippery so I know what to change?