It's very straightforward...
Assuming you've been approved as a contributor, simply compose an email with the subject line as the title of your Project.
Attach your images or video, or embed them in-line with text in-between, if you prefer, and send to post@aopprojects.posthaven.com.
Oh, you can add #hashtags at the end of your subject line, too.
In terms of the size of your images, think about the screen 'real-estate' that you have to play with. Very big, vertical-format images, for example, may require scrolling to view, so re-size your pictures accordingly.
Lastly, think about providing some context for your work, if appropriate. An 'artist statement' or piece of writing about the work can help here, but only if you feel it needs it. Otherwise, put as much or as little text in accompaniment , as you like.
That's it!