I definitely can't figure out how to post pics from my phone.
The mobile site is very basic. On the bottom of the page there will usually be a link to the desktop version of the site. You will have to click that link and it'll reload the page as if you were viewing it on a computer. The page will be a bit more cramped but a link will appear allowing you to add attachments to the post.
You can also usually access the desktop site via your browser. Almost every browser on every mobile device should have an option in their drop down menu to view a desktop version of any site they are on.
For Safari on IPhone it will be an option if you click on the Aa button in top left corner.
Android, chrome type browsers it'll be an option in the 3 dots menu, upper right. Or at least that's what it is for mine, I use Brave browser.