I am pretty sure you can add more planet images. Have a look at events_imageList.xml in data.dat. The contents should be pretty self–explanatory.
Thanks chronial, I wasn't aware of that file. I hadn't looked at the xml files at all, so far. So, in this case I will add more planets and backgrounds, so it gets less repetitive. This means the release will be pushed back a little, I'd say later next week. I want to make sure everything looks good.
As for the other mod (replacing the outline-style font on the buttons) it will take me longer. There are plenty of buttons and I think I will release that seperately because I am not sure that everyne who likes to have new planets also wants the font upgrade. However, if the new fonts are liked by the majority here, it might make sense to release it together.