Here's an example of a puzzle activity built from scratch.
Create a folder called "my_activities" as a subfolder of omnitux/data ("C:\Program Files\omnitux\data" on Windows)
Save a single image called "my_picture.jpg" in the folder "my_activities" created during step 1.
Download (right click - "save link as") this example XML file with comments in your "my_activities" folder.
You can check/modify its contents with a text editor (basic editors like wordpad are ok, but it is better to use an editor doing XML syntax highlighting)
Make sure you select the English language.
Your new activity should automatically appear in the puzzles menu.
The easiest way to build new activities is to copy and then modify some existing XML files from the "omnitux/data" folder.