How to make a TP (transparent) Sprite BG?

[Phantom-Blade]

I want to port a wad (Not sure if port is allowed in the website) But when I put a sprite it came up with a colo(u)red background. I tried Paint.Net. But that left me with no transparency of the coloured bg.
6708
Someone please help!

[Colby]

1. Ports are allowed on Skybase
2. Those pictures are currently in BMP format, not DoomSRB2 format, plus you might need PNG format so that you can have the transparency you want.
3. You can also make the background a certain color, then have SLADE remove the color when you convert it into DoomSRB2 format.