Click "Load image" and select the button image file you would like to add to the theme. It will be opened in the dialog.
Here are three state images in the example, so set total images" field value to 3.
To define the button's hot state appearance, set the "State offset" fields to 0, 1, and 2. Then the topmost state image will be set for the Normal state, next image - for the Hot state, and the last state image - for the Pressed state.
Press "Create button" button. Created gif buttons will be added to the current theme and will be opened in the Theme editor.