Please help with canvases

Guessing you mean that you are trying to make an easel painting? If you are I suggest you look at This Tutorial. it's not an easy job but the results can be worth it.
If on the other hand you are trying to just replace some of maxis dreadful paintings on the easel then you need to make a .package file as an override, search for the paintings you want to change, put your choice over the original to the left with the large painting, from the top, leaving that small gap at the bottom for the medium and cover the small completely. Save and try it out. It's a long search for the originals, (Tools/Sims File Cruiser/check dds. at the top, then thumbnails at the bottom and begin the search, export 1 at the time give each one a name you will recognise-ie,1 large lady) there was a beautifully easy set of instruction I used to do mine but I just can't find the site. I hope something here is helpful to you.