Set the desired condition and press the "Add" button.
If there's a condition you don't want, click on it in the table and press the "Delete" button.

Warning!
Earlier conditions (higher in the list) take priority over later ones (lower in the list.) For example: if you defined "< 1024x768 Center" and later "> 800x600 Stretch", all pictures with resolution > 800x600 will be stretched, and pictures <= 800x600 will be centered even though they're not specifically mentioned, because of the second rule.