Does anyone know if photos are restricted in size?
Also, can captions be added?
I don't know if CM limits the image size, but you generally want to keep your images rather small anyway because many users have file size limitations on the email they receive. Your best option is to optimize the file as much as possible, add it, and run a test.
In regard to the captions - you can create your own using html and css. You may run into trouble using their editor to add the images, so you may want to embed them directly in the template or the description code. In the description editor there is a button that says "Source" - if you click on that you can edit the HTML and take care of this from there.
users file size limitations should be irrelevant unless you attach them to your emails, wgich is not recomended anyway as it increase spam rating for some ISP's