Do you mean in gMail's webmail...
Or in Apple's Mail>gMail account...
Try this, not sure what OSX version you're on or which versions it works on...
New message composition window
Drag in the logo graphic file
Edit » Link » Add, paste or type the full URL uou want
Then select and copy the linked graphic, and past it into a new signature on the Signature tab of Mail's preferences panel.