Why is it that when I'm printing photos, they come out clear, but when I print drawings and oil paintings off Google Images, they come out slightly pixilated?

Images designed for screens typically use 72 dpi (dots per inch), whereas print typically uses 300 dpi. This could be part of the explanation.

Google saw you coming, and made the picture a thumbnail ( a small and lower resolution image ).

That so @jca could not print and blow the image up to an 8 X 10. jk

To conserve space images may be saved at a lower resolution than the images you scanned. Try printing them a little smaller and the pix’s won’t be as noticable.

They haven’t been vectored for your use. Could be that what you’re using isn’t for general use because you haven’t paid for the rights.

It could be a difference in resolution. Most of what’s on Google Images isn’t at a resolution that will print well. DPI is another answer, as @gorillapaws said. I second that emotion.