Try setting Whichslot to 0 before the foreach loop begins.
OR
You could set it to 0 (or wherever you need to start drawing the images) on the False node of (if Index != 0)
Anyway you do not need Whichslot, you can simply multiply the current index with 50 which will give you the same result. Unless ofcource, you intend to start drawing it on X greater than 0.