Personally, I use Microsoft PowerPoint for most of my creations, and sometimes switch out to Inkscape for bigger projects. Unless you're willing to pay Microsoft to get all of Office programs, using alternatives like WPS or LibreOffice should still provide the necessities to place shapes around and color them. Other people had also used Google Docs (online) or Adobe Illustrator, though I'm not sure how they exactly work, but you may give them a try as well.
In case you need some pattern for the tanks, some assets/spritesheets are out there as a guide, so that your Fanon tanks stay closer in design to the actual game. Here'stwo you can download and use.