Millisecond Forums

I have several pictures of 5 different categories, each category containing 7 pictures. I struggle to find any way to let the program just pick one picture out of the seven pictures randomly. So that every participant is presented with just one randomly chosen picture out of the category. I know how to show them all randomly or in a row, but I am not able to figure out how I can restrict this to just one item. 

If anyone has any idea, I would appreciate this a lot! 

<item As_forced>
/1 = "As -3"
/2 = "As -2"
/3 = "As -1"
/4 = "As 0"
/5 = "As +1"
/6 = "As +2"
/7 = "As +3"
</item>

So here is one example of the category "As_forced". I want that every participant just is presented with one item out of the seven items. 

Thank you very much for your help!

Cheers, Clara