Well, I didn't use it because I didn't write it. This is Wonko's app. I just happened to suggest that he expand it into this from the binary clock.

As for why... Well, I am not an expert, but one, when this started out as just a binary clock, drawing circles with Cairo was a lot more processor efficient, and easier to code, than accessing system icons (there also aren't circle icons of the type the binary clocks were using, so it's a lot more space efficient to just write in a Cairo library or two with python bindings and some python files, instead of sticking png files in.

Even once it took off, it was easier to use Cairo, because it allows you to render text (in different fonts), shapes, and just about anything, dynamically in any color the screen can render (realistically speaking; I'm pretty sure human perceptions stops being able to distinguish colors before the code stops being able to specify the difference). A PNG on the other hand is static, you can't edit it at run-time - at least, nearly as easily. And because a png is a raster graphic, it (usually) takes more processing than vector graphics (which is what Cairo uses).

Anyway, yeah, I understand your frustration. I'm dealing with the same thing in trying to figure out how to use the png right now. But hey, that's life. For the things that matter, you figure out how to acquire the skills necessary to make them happen, if possible.