Are images from the Internet high-enough resolution to print on canvas?

It depends. Many images online are low resolution as they are created to transmit quickly across the Internet. 

If you are doing a Google image search for images to print, we encourage you to search for High-Resolution images. To do so, select "Tools". New options will appear to search, with the leftmost option being "Size". Select to search for "Large" sizes, as these will be higher resolution.