While you can use an image editor to manually select and copy images from a sprite sheet such as the one attached here, you might want to try this tool.
After running the utility and selecting the images to download, each sprite was numbered 0.png, 1.png, 2.png... which has names compatible with App Inventor.
Those individual images file names can then be placed in a list and used to animate an ImageSprite (see:
General Animation Snippet).
It is easy to use and the video explains how to fix problems that might occur.
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Happy Inventing!