Be sure not so save sub-pictures in JPEG format. When you re-open the JPEG file, the pixels will not be exactly the same as before.
Also, converting an image to GIF can have unexpected effects. For example, the following sequence does not work:
The problem is that converting a JPG to GIF tends to change the colors in the image slightly, and the colors might change differently for the smaller image than in the original image.
The following normally works:
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