How do you even get Irfanview to output lossless JPG, anyway? I've been using it for years and I've never noticed that was an option. Anyway, I know you can also get around this issue by opening your image in Internet Explorer, and setting it as your wallpaper from there. It looks like that just copies it as a bitmap with no compression.
Renaming your BMPs and JPGs definitely seems to do the trick though. Thanks for that! Now I can use a rotating wallpaper again if I feel like it.
Back on topic, I found this wallpaper on Bronibooru, and I've been using it on my phone (which uses a 640x480 wallpaper). It aligns nicely so that I don't have to cover up any of the faces with icons, and when I'm bored I can just scroll back and forth looking at ponies.
And this is the one I use on my netbook. I always have some program or another occupying the screen when I'm doing stuff, but it's a nice image to start the day with.
And for my logon screen I use this one that I'm too lazy to upload to imgur.http://nzdjh.deviantart.com/gallery/#/d4otdr5TechRepublic has an article about how to do this on Windows 7.
Do you have one of these for Pinkie too? These 5 don't align with the one in the post just above yours. I must have all of them and they must be consistent.