Good idea, Jack. Count with it for the next version.
However I am a bit concerned about the possibility of making a mistake and selecting Save All instead of Save As, for example. Many times one is simply playing with an image and don't want to save what is being done. Yes, this is bad practice (one should always play with a duplicate of the image, never with the original), but this does happen.
So how about a confirmation message? Something like "Do you really want to save all images?" when the user selects Save All.