Is there a tool or browser add-on which will overlay images with their filename and/or alt tag? Without needing to mouse over each one.
Something like this made up example:
The Web Developer toolbar has items under its Images menu to display the image alt, dimensions, path(which will include the filename, but there's not an option for just the filename), filesize, and a lot of other stuff. There's also an item(View Image Information) that will pop open a new tab with just the images from the current layout, and a bunch of basic information about them.
The items to display properties don't overlay exactly as you describe. They tend to push the images around a little because they're placed into the page, but otherwise the toolbar does everything you want and a lot more. If you're not feeling terribly picky about the overlay, I'd just go with it.