In the line of links just below the credit line on the photo page are one or two yellow 'thumbs up' icons. Just one, which says [👍🏻 Like] if there is no description. Or one saying [👍🏻 Like Image] and another with [👍🏻 Like Description]. You can also find the 'Like' thumbs on the 'Full listing + Links' format of the photo search under 'Thumb'.
Click on one or both to show your appreciation.
When this happens to one of your photos, it is reported on the section of your profile 'Use of my photos around the site'.
Simply click them if you like the image and/or description (separate entries in the thumb pop-up for each).
We don't know what use we will make of the data, but note that there are number of things we won't do. We won't disclose who is voting (all anonymous), we won't be using it to produce leaderboards, and we won't be disclosing which images that have few/no votes. The general idea is to simply find great content worth showcasing.
If you find a picture or its description particularly good or useful, you'll find links to heap praise on either in the thumbs pop-up menu which shows when hovering over the thumb symbol to the right of the main picture. Just click the appropriate link(s).
You can also 'thumb' your own images to keep track of your own favourites. This is counted separately from feedback by others.
While you're at it, you can use the thumb pop-up to throw the image in one or several 'image buckets' - categories which classify the type of image, e.g. whether it is a close-up or a panorama, or if it shows people or landscapes. By doing this, you can help Geograph to narrow down searches and add value for other site users.
Click the link 'Use of my photos around the site' near the foot of your profile page.
Follow the link at the top of the page you arrive at to get notification emails of new additions in selected areas.