If you are in the forums and you see an image that you think would be good to have added to the database, you can propose that it be added.
Click on the image and that'll open the fullsize image in its lightbox.
In the lightbox, right below the full size view of the image, is a link that says, "[Propose image for the plant database]"
Click on that, and follow the screens. You'll have to search for the plant, select the plant, then choose a gallery, then confirm. It's easy peasy.
The only real requirement is that the plant has to already exist before entering this. So if the plant doesn't exist yet, you'll need to add the plant first, and then wait until the search engine finds it (which could be an hour or more).
Your proposal will be reviewed by the moderators. If approved, you get half an acorn and the owner of the image gets 1 acorn (per gallery per plant).
The owner of the image is, of course, fully credited as the owner of the image just as if they submitted it directly themselves.
If you have any questions, let me know!