I have solved my other problem, but now I find that the images are too light against the white background. It seems that Window Blinds is drawing them with some transparency, even though the images I provided already have transparency.
I took the images from the Windows 7 theme (i.e. straight out of the .msstyles file). Each frame has a blue gradient that varies from 7% to 20% alpha. After adding the image to my theme and applying it, the items in a list view look way lighter. I have to adjust the contrast/brightness on the images for them too look normal.
Does anyone know what's going on? Can I tell Window Blinds that I do not want it to draw these images with extra transparency? To me this seems like another case where Window Blinds provides less, not more, freedom when customizing themes.
Note: I have tried with images that have pre-multiplied alpha, and with ones that don't. I get pretty much the same result.