How to control the size of images and graphics?

Question

I've been having trouble arranging graphic elements correctly. Here's an example:

im1 = Import["http://upload.wikimedia.org/wikipedia/commons/5/5c/London_2010_Tower_Bridge.jpg"];
GraphicsRow[{im1, ImageAdd[im1, Graphics[Disk[]]], Graphics[Disk[]]}]

The circles are both specified in the same way, but show at different scales, so the diagram that attempts to show how it works isn't very satisfactory. Obviously one is scaled to match the image, but I can't see why they don't appear the same size. Row is similar to GraphicsRow but gives me the same problem.

Answer 1

The problem occurs because in the display, the disk used in the mask has a diameter equal to the height of the image, whereas the stand-alone disk has a diameter equal to the width of the image. You can correct this by specifying the size of the disk explicitly.

im1 = Import[
   "http://upload.wikimedia.org/wikipedia/commons/5/5c/London_2010_\
Tower_Bridge.jpg"];
disk = Graphics[Disk[], ImageSize -> ImageDimensions@im1];
GraphicsRow[{im1, ImageAdd[im1, disk], disk}, Spacings -> 0, ImageSize -> Full]

Answer 2

Another option would be to wrap the images in a Pane with automatic width and fixed height, e.g.

Row[Pane[#, {Automatic, 200}] & /@ 
 {im1, ImageAdd[im1, Graphics[Disk[]]], Graphics[Disk[]]}]