Here I used the 6x6 slots, and matted my 4x6 photo on 6x6 paper (lowerright.)
The other side of my 6x6" slots page. Notice I had to cut the map to make it fit across two slots.
This is a terrible shot, but hopefully you can see that the vertical picture of the Epcot dome is cut to fit down two horizontal slots, and matted to fill out the slots.
I had a horizontal 4x6 photo, and four 3x4" slots to fill. So I chopped the picture up and matted it. I think chopping off-center givees a better look than choping it equally.
Another horizontal 4x6 cut. and matted.
The flowers in the bottom middle 3x4" slot are cut from a larger photo. I would have used the whole thing, but I needed to fill that slot and was able to crop it down.
Vertical photo and vertical journaling, chopped across the middle to fit the 3x4" slots that run doen the middle.