@Kamphund: If you need to expand your canvas in Photopea, go to "Image > Canvas Size." You can multiply the height of your canvas by the number of frames you wish to make your canvas/image. My images are 659 pixels (if my memory serves me right), so if I want two frames, I adjust the canvas height to 1318 pixels. I typically scale my images a little larger than the canvas boundaries, in one direction or another, and let images overlap to prevent blank space. 

Sorry, I feel like I'm overloading the chat. Hope my earlier tutorials were helpful.