If you want to swatch or switch pictures already on your poster or page you can do that pretty simply. This can be done by clicking on the photo you want to change and you will see on the image is a icon with two squares and an arrow (found in the upper right corner).
When you click on this it will allow you to left click and drag that image to another photo or frame on that page or poster. You will see blue will highlight around the photo.
You can then release your mouse and it will switch these two pictures.
If you want to replace images from ones you have found under Ancestry Records or from your MyPhotos tabs you can go to this link for instructions: https://help.mycanvas.com/hc/en-us/articles/360007752931