That would be a great feature!!
The 57x57 when trying to create a block that needs a bigger photo, that is way too small, and even when I upload a 500x500 image it makes it the 57x57 thumbnail size in my block.
I get around it by changing the 57x57 size in admin under system, setting, appearance, then go way to the bottom, find Grid image width and height to whatever I need and then change it back when I am done creating my block. If you don't change it back to 57x57 your product images in admin are huge and you can't see them ... just curious why is the block image upload connected to the product grid image?
PS. You can also FTP the image up and manually add the url link to your block, but for my clients they wouldn't have a clue how to do that.