You can control that by doing the following...
- On the Main Page click on User CP on the left near the top in the Main Menu area.
- On the User CP page click on Edit Options on the left.
- Go to the Thread Display Options area near the bottom of the Edit Options page.
- You can set the action to be taken as well as the allowable dimensions (height and width in pixels) before the specified action takes place.
Carry on.