If you have the Workflow app installed and want to make photo grids, you can easily make them with a workflow.
Run the workflow and choose a number of images. When done you will be presented with a collage. The images will be cropped to squares if they have another format.
This is a workflow I found on the Workflow site. The only thing I have changed is the spacing between images. In the original workflow there were none.
Download the workflow here: