I want to submit a photo for this week's contest, but I am not certain how. So, how, can I? And how big must the photo's maximum size be?
Stack Exchange has a built-in image uploader, which is easily available as soon as you start writing a post:
It supports images up to 2 MB, so you might want to resize your photo first. You can embed larger images if you host them elsewhere, but the benefit of smaller images is that users visiting the page don't need to download several MBs of photos. A possible downside is that (like all written content on Stack Exchange) it will fall under the CC BY-SA 4.0 license.
Best of both worlds: embed a smaller version of the photo in your post, and make it a link to the externally hosted unmodified original (well, of course RAW files won't work). For example, the following syntax
[![enter image description here](https://i.stack.imgur.com/xZl6H.jpg)](https://i.ibb.co/nbm07NF/leeuw.jpg)
will produce this:
For more details about this syntax, see What's the recommended syntax for an image with a link?
Unlike the old Featured Photo contest that limited images to 275 by up to 310 pixels, the new site layout's featured Photo of the Week sidebar doesn't limit image dimensions. So you can upload any image size (in terms of pixel count) you wish. The only limit is the Stack Exchange uploader's file size limit of 2 MB.