That means your photo that you are trying to add to your sig line is too large.
Avatars and signature block photos have size limits and the software does not auto re-size what you upload.
A photo editing software package will allow the "canvas" to be reduced in size to fit the forum software limitation.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW