I'm sorry, but I must be a bit dense because I'm still confused. So here goes.
Irfanview has been my default imaging program for some time. After creating a D520 skins folder, installing the HD default skin (skin1o.tgb) into the aircraft folder (and overwriting the existing one), then loading the alternate skins, I ended up with a D520 skins folder that includes the alternate skins as .bmp files, for example, "bulgar2_2048.bmp", "Vichy.bmp" etc. Hence the skins are already 2048x2048. (There's no "default" .bmp in the skins folder, of course.)
After opening up Irfanview, and browsing to a skin (.bmp), an option under "Image" is "decrease color depth". This shows the default .bmps as "256 Colors (8 BPP)" but the only choice is to lower the colors and BPP. Is this what I want to change? I don't think so.
I did find that I could install and use the non-HD D520 skins available here at the Skin Shop provided I went back to the "original" default skin file. But otherwise, I get a pink default skin and the alternate skins don't appear as options when customizing the D520.
I hope I am explaining this correctly. Thanks.