Hi! For reference, my signature image dimensions are 450x100 pixels — however, the maximum height for the forum signature is 190px. I’m unsure about the width dimensions exactly, but I will add that information once I know since I’m a little unsure myself. What I did was test a bunch of dimensions until I was content with it.

As for urls in the background image, I may need more information. I almost always use a direct link to where I’m hosting my image. But, if you’re already doing that, you will need to put some kind of text in to reveal the image being uploaded into the signature. As of now, the signature “expands” to the amount of text in the actual signature box editor. If you haven’t tried, try including:

[transparent]text here[/transparent]

And just fill it up with whatever. I usually line break after one character to just expand the box so I can see what’s being uploaded.