Can someone tell me how I would go about posting a photo on DU?

[usonian]

3. The semi-official post on how to do it is here:

https://www.democraticunderground.com/103659987

That said, I usually use postimages.org lately.

I select "640 by 480" size, to even up large images for posting, and drop my photo on the web page.

Use the "direct link"

Optional, for extra credit:

External images: If you run into an image link that DU won't render, or some "webp" or other format (it happens a lot to me) then ask in the photography forum https://www.democraticunderground.com/?com=forum&id=1036

because converting formats and the like is going to depend on your computer or device.
Sometimes easy, sometimes nuts.
And there are online sites that will convert images, as well.