> What format do these renderers use then?
> Just BMP? Or also PNG?

I haven't seen PNG either, but I imagine that some might be able to export that.

These are the most common outputs:

tif, bmp, tga, jpg, avi

tga would be a nice one, but I heard the format isn't especially simple.

avi with no compression is OK too, but it's essentially just bmps in one file.

meselfs himself!