Signature image problems

Hi, we on the SuperTuxKart team are using signature banners linked from our website. They work fine for those of us who already have it, however when putting it as a signature for another forum member or editing our own signatures we now get this error:
I assume this started to happen after a software upgrade on the forums, or eventually after we started hosting our website ourselves, however even after pointing to the correct image it doesn't work:
Any suggestions or fixes? Thanks a lot!
It was not possible to determine the dimensions of the image.
I assume this started to happen after a software upgrade on the forums, or eventually after we started hosting our website ourselves, however even after pointing to the correct image it doesn't work:
- {l Code}: {l Select All Code}
[img]https://supertuxkart.net/images/9/99/Stk_dev_banner.png[/img]
Any suggestions or fixes? Thanks a lot!