Select the Custom Package style, instead.
In the Image Settings panel, enable the Photo Border if you would like to have a grid-like division between images. The border takes on the color of the page background. You could choose Inner Stroke if you want the border to be a different color than the page background, but a word of caution; I found that changing this color resulted in pale outlines around each image, unless the images are perfectly aligned. Not sure if this is a bug, or just me, or it may just come down to the fact that the Print module wasn’t designed for this level of accuracy.
I went with a border of 6 points. File Resolution means NOTHING unless you're printing this. The file size when saved as a jpeg is the same, whether you pick 150 PPI or 300PPI.
The default page dimensions are 8.5 x 11. A square format seems to be very popular with these collages so let’s change this before we start adding cells.
1. Scroll down the Panel group to Print Job.
2. If necessary, enable the Custom File Dimensions.
3. Click on the long measurement and change it from 11 to 8.5. Now you have a square format for your collage