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 and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW