
Since, of course, Preview does not offer any proper image editing tools and no way to exactly position the two images, the ImageMagick solutions posted here are preferable. (Or why not just install MacPorts' GIMP package while you're at it and enjoy a fully equipped image editor?) Paste the copy of the original A and move it manually to the left.This will result in a stretched version of A. In the Tools menu, select Size and adjust the width to the sum of the width of the two images (in your example, 640 px).This solution uses only Preview and may be handy if you can't install software on a machine.


Here is a link to a useful and clever tutorial. Another way is to create an HTML document with the pictures then. Then create a new empty image at the size of both images combined then copy+paste them into the empty image and save it. However, if you do not mind Preview, which is part of OS X, you can do it for free. What I do in Paint.NET or Microsoft Paint is to calculate the pixel size of of both images - you'll need both height and width.
