I went to post feedback about Windows 10 photo viewer, and wow, so many others have complained about the black background.
I've found some articles on changing the background color from black to something lighter via the registry, but so far, they only work with older versions of Windows (7 & 8, 8.1.) If I find a solution, I will post it here.
Meantime, I went into Picasa, clicked Tools, Configure Photo Viewer, and set it as the default program to open PNG files (which includes all masks, alphas, and embellishments.) This works. However, when I try to make it the default for JPG files, I haven't been able to make this setting "stick." Something in Windows registry overrides it.
However, if you open a PNG file in Picasa Photo Viewer, you can scan through all images in a folder with it, including JPGs.
Will let you know if I find more ways to modify the systems in Windows 10. I like most of Win 10; this is probably my only real gripe.