Values you can change in the Images option of the configuration menu
1.  Small Image Width
2.  Small Image Height
3.  Heading Image Width
4.  Heading Image Height
5.  Subcategory Image Width
6.  Subcategory Image Height
7.  Calculate Image Size
8.  Image Required (use true when testing and developing, and false when your store goes live)

How To Change A Value
1.  Click on the name of the field you wish to edit.  It will change colors letting you know its selected.
2.  Click on the "Edit" button on the right side (see Figure-1)
3.  Make your changes
4.  Click on the "Update" button to save your changes (see Figure-2)

Figure-1 Click to enlarge

Figure-2 Click to enlarge