Why you might do this: to intensively study two pieces of comparable line-art for small differences. The process can be easily encapsulated into an automated Photoshop Action.
1. Open your image in Photoshop (here we use Photoshop CS6). Select the Crop tool in the Toolbar, and set a crop that re-sizes the image to the required size.
2. To the top Menu, then: Layer | New Layer from Background.
3. Open your second image. Run the very same Crop and then copy-paste the picture on top of your first image. In your Layers palette you will now have two layers…
4. Now go to the top menu, and turn on the Timeline (Menu | Windows | Timeline). The Timeline will then open at the bottom of the screen.
5. Ignore all the stuff over on the left in the Timeline. Go over to the far right and find the tiny almost-invisible ‘little down arrow and lines’ icon. This lets you access the “Make Frames from Layers” command.
6. Once this command is activated, all your Layers are auto-placed into the timeline.
7. Pressing the “play” button on the controls will probably show that your animated GIF is flickering too fast. Click on the “O sec” under each thumbnail. This opens up options that let you fine-tune its playback speed.
8. Done? OK… File | Save for Web (do not choose any other type of Save, such as File | Save as…).
9. Open the resulting GIF and view it. Even if you have a GIF blocker addon for your main Web browser, the standard Windows Internet Explorer browser can open them and play them.
10. That’s it. Upload the GIF to your blog, forum, etc. as usual. Many people now have GIF blockers in the Web browsers, so you may want to add a note that the picture you’re posting is meant to be animated.
Above is a GIF that compares the slightly different line work on two bits of line-art.
2D digital art and painting 