[thirteen]

Open your applications folder and locate the fulltilt app. If you right click on the fulltilt app in the finder and select "show package contents" you'll get a new finder window showing a "contents" folder. These are the contents of the fulltilt app. Now go inside of the content folder, you'll see a "resources" folder. Go into that and you'll see the "Graphics" folder where all the images are contained.

Now simply replace the exisiting ones with the images from whatever mod you downloaded. (windows mods will work, the graphics files appear to be the same).