Pic Style - Checkerboard от coyoteelabs

With this add-on you can easily view if the image you are currently viewing (using right-click menu -> View image) has transparency.

It does this by changing the simple white background that is shown by default by a checkerboard background (similar to the ones used in photo editing software).

Permission explanation:
"Access your data for all websites": The add-on checks if the current webpage is a standalone image and if it is, it will inject CSS to add the checkerboard background to the image. In order to check if each webpage is a standalone image, it needs to "access the data" of the websites. No data is read or stored from the websites.