Do you want to fade two images with each other? Or fade the current image to nothing and then fade the next image from nothing?
Use alpha in tocolor to fade it. (red, green, blue, alpha).
dxDrawImage(0, 0, screenSize.x, screenSize.y, backgroundTexture, angle, 0, -120, tocolor(255, 255, 255, 255))
To fade it a bit
dxDrawImage(0, 0, screenSize.x, screenSize.y, backgroundTexture, angle, 0, -120, tocolor(255, 255, 255, 100))
Now you need to add or subtact a bit of alpha every render. If you add 1 alpha, the effect will take 255 frames, so about 4 seconds at 60 FPS.
Let me know if you need code/explanation.