Probly because you added the pic on top of the text in your sig. Max pic height that you can have along with a SINGLE line of text is 110 pixels, that image is 115 pixels. Sorry I should have told you about it....
Here is the same pic sized down to 110 so you can have that line of text still. You should be good with it.
But I could be wrong, been told to leave the rule explaining to the mods. So please PM a mod to ask and make sure you wont be breaking another rule before you try again.....