I've be using a combination of "painting" apps on my iPad to create digital "paintings" out of my photos. I'd like to know what the digital specifications of both the initial photo AND the painting should be in order to print them on relatively large canvases (perhaps something around 24 x 36" or larger.) I was told that the apps on the iPad cause a loss of resolution in the final product but I would like to sell my images to catalogs and want them to be done properly. I now shoot with a 24 MP DSLR but a lot of my photos were shot on a 10 MP Nikon D80, I think at the finest resolution possible. Would need to double check that.
Your going to need to around 300 Pixels Per Inch (PPI) for higher quality print products. See the guideline below. However, you mention 'canvas'. If that is your substrate material, then the relatively course structure of the canvas may limit the printed details and you could get by with a lower PPI.
There is a lot to say on this subject, but start with these guidelines in the link below and know the difference between a printer specification (Dots Per Inch) vs the printed image resolution (Pixels Per Inch)
You will need to uprez your images to reach your desired print size of 24x36
You can see that a 24x36 size print, printed at 300 PPI, would require about 78 MP.
Hopefully this helps clarify what you must do to get your large prints.
Also, I sometimes print at lower resolutions because I know my audience won't be standing too close to my print.