Button and thumbnail background images don't work in high contrast mode

[chenba]

When the OS is in High Contrast mode, Firefox does not display background images. But Screenshots use background images in the addon and on the web site. Here's what My Shots looks like in Windows 10 high contrast mode:

And here is the addon in action:

[chenba]

For the thumbnails, if we generate the thumbnail images the way is done in PR #3712, then we can simply switch to img tags.

[chenba]

(There will be some work required for existing shots without thumbnails though.)