You can externalise the images, shipping them as separate files or having your app download them from a Web server. WPF is happy to use a HTTP or file system URL as the Source for an Image element.
But this just shifts the problem: your initial EXE is smaller, but now there's a bunch of other files to download as well. What it comes down to is: if you've got 12 MB of images, you've got 12 MB of images, and those 12 MB have got to get to the user's machine somehow!
If you're concerned about a 12 MB application size, you could try to make the files smaller -- lower resolution, different file format (e.g. JPG instead of BMP), etc. Or, if the user isn't going to need all of the files, you could have the app download them on demand (but note this could create delays, or errors if the user is offline).
On the other hand, by modern standards, 12 MB really isn't all that enormous for a game-type application (which is what I understand yours to be). So unless you're targeting a market where a lot of users are still on dial-up or other slow or poor connections, it may not be that big a deal.