You can see an example here: it's a Google Site page in my class project (like my students, I have a Google Site for my project) -- Random Hanuman Art. On the left you'll see a Hanuman artwork from South Asia, and on the right a random Hanuman from India, with new art at random whenever the page reloads.
Here's the same pair of widgets running a Canvas page: Hanuman Widget. Same here as in the Google Sites page: new images at random each time the page reloads.
So, what is the secret? The secret is to host a very simple HTML page with nothing but the script call in your own webspace (or, in the case of Canvas, you can host the page as a Canvas file... at least for now; I worry, though, that future security measures may remove that option).
It sounds weird, but once you get used to the workflow, it's totally doable:
STEP TWO: Create HTML page with script. Then I create a simple HTML page which calls the script. This is the HTML page that calls the script of Indian Hanuman images. If you click on that link, you will see the results of the script running, and the script is running there on my server. The results that script returns are just HTML. If you look at the page source, you can see the script call.
STEP THREE: Embed the HTML page. In Canvas, the way you embed a page is with iframe, but with Google Sites it is even easier; you just choose the embed option and paste in the URL of the page; it even gives you a little preview so that you can know what to expect:
I learned about this workaround from Sharmaine Regisford and James Jones when I first started using Canvas a couple years ago... and — doh!!! — I finally realized the same solution works for Google Sites also.
P.S. This is the same workaround that makes Twitter widgets possible in Canvas. More about that here: Twitter4Canvas.
P.P.S. This is why I think everyone needs a Domain of One's Own where they can host their own (distributed, dynamic) content. My school uses the awesome Reclaim Hosting: hosting by educators for educators! At my school, it's branded as OUCreate, and it's free for students, staff, and faculty. Reclaim + Canvas: it is a powerful combination.