any 700x825px pic can be used as a static background. full screen .swf backgrounds should be 1122x825px minimum
find pics to use, or draw your own in an art program that can save .png/high quality .jpg files. there's a
perspective grid in another thread that can help with that
https://www.undertow.club/threads/tutorials-templates-summary.15972/
download one of the .fla templates from "To make backgrounds: > SWF backgrounds templates"
you will need adobe animate / flash to open them. which file you get depends on how big you want the background to be.
(regular is windowed, hd is fullscreen, xl is widescreen)
https://www.undertow.club/threads/how-to-make-an-swf-background-static-animated-and-or-16-9.9308/
this guide is still good to follow. you just need to import whatever image you plan on using to your library, edit the 'Background Template' movie clip, and click+drag the image from your library into the scene (make sure you have the 'Background' layer selected)
after that, it's pretty standard to other .swf mods. you click+drag the movie clip to your main scene, give the instance a name and then edit the 'Settings' document to registerMod("instanceName"), hit publish and import it ingame.
you need to publish the .fla from the same directory as the SDTMods folder with ModElements,ModPackage,ModTypes .as files inside