The 25k thing is file size, when people say "too big" it doesn't mean the actual picture size as you see it, but how big the file is. There are ways to compress file size and so on, I know it's confusing, I used to not really understand it all myself, but there are plenty of people willing to help you out, you can always ask them to make the file size smaller.

As for the limit, it's not actually posted anywhere, you just kind of find out by word of mouth, so don't feel bad about it