If you have two separate files of the same exact image, the best way to compare the quality of the two files would be to use Adobe Photoshop. You can do that by following these short steps:
1. Open both of your image files in Adobe Photoshop. It's important that the resolution is the same for both images as you import them.
2. Copy one of the files and paste it onto the other file. This will create a second layer, one image laying on top of the other.
3. Zoom in at 100%.
4. Then, continuously hide/unhide the new layer that has been added (Layer 2). This will allow you to see the quality difference between the two image files.
Note: In some cases, the two layers may not be perfectly lined up. To adjust the layers, you can select the Move Tool and use the Up/Down arrow keys on your keyboard to align them.