Can you post a picture? Your sizes don't make sense to me. It sounds like the finished size of the fussy cuts is 7" (from 7.5" cuts). And the solid and HST pieces are 3.5" finished. Cut at 4" for the solid, and I guess 4.5" squares to make 3.5" finished HST from.
Assuming that's all correct, then what you need depends again on your panel. You said 6.5" pictures - I will assume that is the cut size - so they will finish at 6" blocks in the pattern. Then you will need 3" finished squares around it. Cut the solids at 3.5". And the HST pieces cut from 4" square (you will have to trim the HSTs down to 3.5" after you make them, so they will finish at 3".)