First, make sure the image you want to use is 111x74px. You can use an image editing tool like picmonkey.com to do this. Now to change the image, go to your page and click on Settings > Apps > click Edit Settings on the app you’d like to update. Then click Change for Custom Tab Image > Scroll over the image and click Edit > click Choose File, and you’re done.
Here's a video showing how it's done.