In general the answer is no but there are some controls that use user supplied images

GImageButton
GCustomSlider
GImageToggleButton

Look at the examples that come with the library (select File | Examples from PDE menus then Contributed Libraries | G4P)
 

Yeah, I saw the ones with the buttons, so there isn't a way (even via code) to use a specific image for the knob.. That's a pity!
Maybe, imho, it could be a nice thing to have in the next version!