Try moving one image set to the following locations and then in the admin insure that the product base image is re-loaded to be from "main" folder

Load your best image quality images to the following

/images/mb06.jpg
/images/mb06_01.jpg
/images/mb06_02.jpg
/images/mb06_03.jpg

As you are using IH2 you do not need these

/images/medium/mb06_MED.jpg
/images/medium/mb06_01_MED.jpg
/images/medium/mb06_02_MED.jpg
/images/medium/mb06_03_MED.jpg

/images/large/mb06_LRG.jpg
/images/large/mb06_01_LRG.jpg
/images/large/mb06_02_LRG.jpg
/images/large/mb06_03_LRG.jpg