BorderTron 3000 is a Windows-based paint program for creating ZX Spectrum programs that draw multicolour images in the top 24 scanlines of the border area.
It outputs Z80 assembly listings, or TAP files (complete with BASIC loader) ready for loading on your real 48K Spectrum or an emulator.
As a paint program, it's a bit crap, but it does the job and it was just a quick Friday-afternoon lash-up. I've included a couple of sample "*.bdr" files in the ZIP that you can load into the editor to see the sort of thing it does.
BorderTron 3000 - Windows 32-bit (827 KB)
It should run okay on linux and MacOS under wine, I'd imagine.
I've also done a version tailored to the SAM Coupe, which is much more flexible if you intend to target this machine.