if the mouse pointer isn't over the image itself (when the URL for an image file), then the resulting context menu is just for the regular content area. this might initially seem trivial, but not so if someone is surfing without a mouse. using the keyboard (shift+F10 for win32/unix, control+space for mac) to bring up the context menu will only bring up the one for the content area, not the image as URL.
is it possible to apply the context menu for image as url for the entire page content in this case?
See also bug 67544, "Save As.. should be removed from context menu for image URLs". I don't think this is an accessibility issue. Most of the things you would want to do with the image (save image, copy image address) are already on the main menus, which are easier to get to using the keyboard than the context menu.
This should be easily doable by just checking document.contentType and if it's "image/something" doing the image context menu everywhere.
gah, realized this bug is invalid --since the menu for "image as url" hasn't really been implemented. the current menu (when pointer is over the image itself) is the same as the one for "inline image". anyhow, filed bug 138074 for the "image as url" issue.