Step 1. First you need to open the photo you would like to make into a square. I chose this instagram photo I took of a page from a vintage Bambi book.
Step 2. Then you need to go up to the top and go to Layer -> New -> Layer.
Step 3. Then a box will appear. Make sure that everything matches the above. You can also name the layer if you want to, but it is not necessary.
Step 4. Making sure you are on the new layer you just created, chose the Fill Bucket tool.
Step 5. Make your colour white, and click on the photo. Then everything should go white.
Step 6. Then you need to select the Eraser tool. Make sure it is the Eraser Tool, not the Magic Eraser Tool.
Step 7. Then you need to make the Eraser as big as possible (mine was at 500), and click in the box. Then your photo will appear through like this...
Step 9. Then go to Layer -> Flatten Image
Step 10. Go over to the toolbar and chose the Magic Eraser Tool
Step 11. Then you need to press anywhere over in the white section, and it will look like the above!
And then you are done, hooray! have fun making circles!