I make them using a primitive image editing tool, it's called "Paint.net". :)
It might be possible to emulate the .NET framework but I've had so many problems with Windows emulators that I don't even bother at this point. I'd recommend looking into it yourself though, because there are a lot of benefits from using Linux. The only problem are all the software limitations, especially the ones I hadn't thought about, like the fact that Microsoft owns the H.264 video standard, etc. I'm about to rebuild my PC and set up a Windows & Linux as dual-boot for this reason.