It doesn't really matter where you upload it, though Mediafire allows up to 100MB (never even seen a picture above 4MB, and that's crystal clear!). Imageshack and Mediafire don't need you to make an account or log in, which is great. I have an account on imageshack, but never log into it these days. Just choose the picture, click upload, there: done! Now I can grab the link for whatever I need, very fast too.
Hope that helped.