Signature images must be no more than a combined total of 500 pixels wide x 350 pixels high, and 150kb. The system will automatically crop pictures that exceed the pixel limits so if you add a picture that is too large part of it will be chopped off.

If you're wanting to add an animated image please check the KB size first, heavily animated images always have a large file size.
To check, right click on the image and select properties, that will give you the info that you need :)

Further information about signature rules can be found HERE.