Hi Andrea, What do you mean exactly by wanting to “change out” one of your photos? Do you mean erase parts of it (if so eraser tool is coming soon), or replace a photo in a collage (if so click Undo until you’re back in Collage mode), or something else?
With cropping, basically you need to make sure the dimensions of the image match the dimensions of what you’re printing. So for example a 16×20″ canvas will need sides in the ration 16:20.
To make this easy, go to Basic Edits > Crop and Choose 8×10 in the drop down for the 16×20″ canvas. This will ensure the dimensions match perfectly. For the 20×24″ canvas there isn’t a preset so you can take the shortest side x 24 / 20 (on a calculator) to give you the longest side’s dimension and type these 2 dimensions into Crop with “No Constraints” selected in the dropdown. Alternatively you can take the longest side x 20 / 24 to give you the shortest side’s dimension.
Hope that’s not too confusing and let us know if you need further help.