Ok, when I make a thread, I have the option to insert an image by using the yellow box with mountains and sun.

It says 31.7, but venture it will change on my next fill up as it should.
But that option is not there when I edit my signature.
I can use the bottom feature to upload the image, but I have to do that every time I fill up my car, if I want the image to be updated.
EDIT: I have since uploaded a new SIG pic, so now they match