All images are editable, and as long as you don't mess with the copyrights of Snitz I don't see why you couldn't.
Of course, you could make your own images, as long as they were the same size, and just name them the same file name...
I chose to forego the use of images for the exact same reason you are - I have a blue background on my site. If you go to my link you'll see it doesn't look that bad without images.
The images you are looking for is already made for you. They are in the 'tools' folder. Each image (the Snitz Forum logos and the login/logout images) come with a black, white and transparent background. They are all in the 'tools' folder.