For custom images, maximum sizes are below:

Avatar = 100 x 100 pixels, 50 KB file size

Profile Picture = 100 x 100 pixels, 25 KB

Signature Image = 500 wide x 100 pixels high, 50 KB

Album photos = 600 x 600 pixels, 100 KB


(You will find some of the avatars supplied on the forum are larger than the allowable custom avatar sizes both in image dimensions & file size.)

As birdy mentioned, it can be a bit tricky sometimes discovering how to post url links for signatures or avatars.

Edit: If you are using url's to link images, you still need to ensure the images fall within the maximum allowable file dimensions.