So, i think that it's impossible for me to change the signature manually.
I don't have access to this database, the server is not my server, I only buy a hosting service.
The goal was to add a picture (coding base 64) into signature code HTML, like this :
--------------040600010806040804032894
Content-Type: image/png;
name="logo_48_10col.png"
Content-Transfer-Encoding: base64
Content-ID: <part1.08070802.03080002@xxx.com>
Content-Disposition: inline;
filename="logo_48_10col.png"
iVBORw0KGgoAAAANSUhEUgAAADIAAAAyBAMAAADsEZWCAAAAAXNSR0IArs4c6QAAABhQTFRF
STg1bGBhhXp6nZKSsauqyMPB39rZ/f/8w54u/AAAAAFiS0dEAIgFHUgAAAAJcEhZcwAAAYEA
AAGBAbqlokxLdrIzKbkP+Z9F/i+W/fP9AUzedZW1i4EwAAAAAElFTkSuQmCC
--------------040600010806040804032894--
Another way to do that ?