Use File >> Automate >> Contact Sheet II and chosoe a folder of your source images.
Change units to pixels as it can occasionally lock up if Photoshop has difficulty calculating how to split a pixel.
You can duplicate all your source images and crop them if needed (set your crop tool to something that divides evenly into 1280 x 720, like 128 x 72). This will also speed up processing for contact sheet.
Contact Sheet is great as you can uncheck flatten layers, and get layers for each one so you can animate them.