FB is currently changing their requirements for news feed images. I'd suggest to change default behaviour accordingly.
https://developers.facebook.com/blog/post/2013/04/03/platform-updates--operation-developer-love/
Minimum is 200x200 (so ppl don't use thumbnail size!) and recommendation 600x600, so default "medium" 300x300 is a bit too small.
I'd use "full" as default.