The signature photo is not like other photos attached to your posts.
The Forum Software automatically re-sizes your images to the maximum size when you upload them.
Not so your signature post.
There is a note at the bottom of the screen above the "upload" button that reads:
Note: The maximum size of your custom image is 468 by 60 pixels or 19.5 KB (whichever is smaller).
To upload a signature photo, you must reduce the pixel count with a photo editor to 468x60 pixels maximum BUT the file size is still limited to 19.5 KB.
It took me several tries to get a photo small enough to be acceptable yet still "look" like the photo I was trying to use.
Hope that helps.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW