The words diptych and triptych come from the Greek language, and were often used as terms to describe a work of art that was divided in two or three parts and hinged together. Nowadays, diptychs and triptychs are used commonly in the photography world to stitch photographs beside each other.
Step 1: Open your first image
Go under "Image" and click "Image Size..." to see the width x height dimensions of your photo.