pvbrowser supports all image formats that are supported by Qt. GIF images should not be used because of license issues. Trolltech may drop GIF support in the future. Use PNG instead. In PNG also transparent images are possible. Also JPG may be used. But here you have no transparency.
BMP images are a little bit special. Only 8 bit per pixel BMP files with a color look up table (indexed) are supported. A color of red=1 green=1 blue=1 will be treated as transparent.