I put paint.net on an older laptop a few years ago and found it to be the next best thing to Photoshop for me. I have Xara Designer Pro and Gimp on there as well, but paint.net is the best for simple, quick image editing (it has levels, curves, layers...). Maybe the U.S. site is different from what you see Inger?, I just see a page with system requirements, language choice and a big download now button. There are also several plug-ins available as with many open-source projects where developers can contribute to them. Anyway...
When using either PNGs or GIFs within VSD, they have to be inserted from the original file for each corner rather than inserting one and copying it – in which case, VSD will save them as JPEGs.