The only way to add bitmap images to warzone map is by tracing them, either manually or by the "trace bitmap" function in Inkscape. Can you give some examples of maps you've seen?
Map with image for a background: 7/3/2020 08:19:42
All images you see in maps are vectors. You can convert images to vectors in inkscape, but the result is usually bad. If you want a background, you can either make it yourself or find an SVG image
Map with image for a background: 7/3/2020 16:18:47
There is a middle east map, iran i think. They have a background that's based on real life. Also, Ollie bye has a map called roads of silk and iron. How'd they get those images?
Map with image for a background: 7/3/2020 21:26:44
The Iran map is pretty obviously a bitmap image that has been traced by Inkscape's "trace bitmap" extension. It's not perfect, but it works (especially for smaller images - personally I'm not a fan of the Iran background).
Roads of Silk and Iron looks like it's just hand drawn, directly in Inkscape (or AI, not sure which Ollie uses).
Edited 7/4/2020 20:41:03
Map with image for a background: 7/4/2020 19:17:01