Creating Collages in Photos and add them to the Photo stream

I have recently moved from an IPad to a Macbook Pro. I used to use an App on the IPad called Pic Stitch. There I could choose photos from the Photos app to create a collage of several photos, and then use "Export" to save it to the Photo Stream. So far I haven't found any way to do this on the Mac. I have tried several Apps. So far, all of them have several issues: 1. They only produce low resolution, and 2. They can't save direct to the Photo Stream. This requires extra steps to export to a folder, add the photo to the Photos app, and then remember to delete the file from the folder.

It seems that apps on the Pad can work directly with the Photos app, whereas apps on the Mac can't.

Does anyone have a solution, or know of an app that will do this without loosing resolution?