You can install the Old Default Image Style add-on (you don't even have to restart Firefox to get it to work)

Thank you! But is there a way to get the old image display without extensions? With an about:aconfig tweak or with userchrome.css?

There's no about:config change. You can do it with userchrome.css (I can't say how off the top of my head). However, all that extension does is overwrite the default image stylesheet and add options in about:config to let you specify a background color. So it should be pretty safe, really lightweight (and much easier to use)

<strike>I don't think that you can use code in userContent.css to fix this</strike>.
<strike>There is no selector possible to set the background-color for the body if there is only an img (body > img:only-child) on that page.
Only a script would be able to accomplish that AFAIK.</strike>

It looks that I'm wrong about this.

• http://userstyles.org/styles/58710/firefox-11-change-image-view-background
• http://forums.mozillazine.org/viewtopic.php?f=38&t=2443535

Add this code to userContent.css in the chrome folder

• http://kb.mozillazine.org/userContent.css

@-moz-document regexp("((.*\\.(png|apng|jpg|jpeg|gif|tiff|bmp))|(^data:image\\/.*))(\\?([^#]*))?(#(.*))?") {
html > body { background-color: #E8E8E8 !important; }
html > body > img:only-child { background-color:#fff !important; box-shadow: none !important; margin: 0 !important; }
}

• resource://gre/res/TopLevelImageDocument.css