mspaint problem "this is not a valid bitmap file"

How are you saving them as .jpg ? Did you select JPEG in the file format options? (I believe paint defaults to .bmp, so you may have a .bmp save with a .jpg extension).

However, as people have already said in the comments, Paint is not a great program for any serious image editing tasks. A good free alternative is Paint.NET, it has most of Paint's ease of use, with a much larger feature set.

I came across this issue today and as far as I can tell it has to do with the resolution of the image. From my testing today, it seems that any image above 1200*1200dpi won't open in Paint, even if it is a BMP file or even if it is only 4MB in size It seems to be purely resolution related. See this thread: https://answers.microsoft.com/en-us/windows/forum/windows_10-files/paint-wont-open-valid-file-types-over-1200x1200/176a85c8-ac6a-4e1a-a6e2-6b7ce7190ea7

Until Microsoft fix it, I would suggest using Paint.net or GIMP. They're both completely free (and ad free) and easy to use with some more advanced features too if you need them.