There has been much of a discussion about the background color of standalone images when Firefox adopted to dark body background for standalone images. Initially Firefox simply displayed a white background (only for the image), which felt out-of-place considering the dark surrounding background, especially while the image was loading. Then it removed the white background all together, making the background of images transparent until they load, and switching to white as soon as the image is loaded. which made the behavior of images like this really confusing.
And then I thought, “Hey!, it’s an add-on thing”, and created an add-on that brings the feature as depicted in the mockups into Firefox.
Thus “Custom Standalone Image Backgrounds” add-on was born. Yeah, the name is a bit long. Anyways, give it a try by installing the add-on from the AMO homepage, or via this direct link. It is still a work in progress, but should work for most of the cases. I will update it as and when I get time
PS: For now, this works only on Firefox Aurora and Nightly (18 and 19)
PPS: I have updated the add-on on AMO to version 0.3, which works with Firefox 15 and above, and also fixes some of the issues mentioned in comments. (most probably the NoScript and lesser distractive in unpinned state)