[Help Me] I can't load a backpack icon in the TF2 Workshop Importer

[ShadyTheFirst]

So, here's the thing. I'm working on a TF2 item, my first ever one, to be exact, and I'm trying to upload a backpack icon. It's working brilliantly so far, except when I try to load a backpack icon, even just a dummy one, it comes up with [URL="http://imgur.com/WcnATfK"]this error.[/URL] Could anyone help?

[Dekania]

I've never used the Workshop Exporter in TF2, but I'm pretty sure you're supposed to use a .VTF file.