how to stop a gallery slideshow

You can’t,
Dynamix WP theme uses piecemaker.swf to run its slide show and by looking at the documentation you can see that they don’t have a none loop setting.

your best bet is to try to replace it with a different flash viewer that has a none loop option like CU3ER image rotator which i know can work great with nextgen gallery plugin for easy modifications.

Hope this helps

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