You can turn picture replacement off or on in MyAdBlock's options.
1. Click the AdBlock button in the browser toolbar and select MyAdBlock.
2. Click the toggle button next to Replace some blocked ads... to turn MyAdBlock on (green) or off (gray).
Deselecting all picture categories also turns off MyAdBlock. Selecting at least one category turns MyAdBlock on.
Your selections are saved automatically. Just close the options page when you're done.