Forum images and signatures cannot exceed 450px in width and 325px in height, COMBINED. This includes all images, links, any text added in the signature, and regardless of images being left or right aligned.
Any text added in the signature counts as 10px height per line. So be sure to include that in when calculating the height of your forum image and signature.
Please note that the forum signature size preview on the Forum Options page is intended as a general guideline only, and is not 100% accurate at all times.
Animated images must be no larger than 5MB.
NOTE: This is not a freezable offense, however, if you receive three Official Warnings for oversized Forum Image/Signature, you will be banned from using this feature.