The main picture on Geograph should be free from annotations because different people may want to use it in different contexts. However, annotated pictures demonstrating features such as geological or geographical details are very welcome. Please upload annotated versions to Geograph's media repository http://media.geograph.org.uk/ and link them from the description of the original image uploaded to Geograph's main archive. This article http://www.geograph.org.uk/article/Howto-Crosslinking-descriptions shows how to make such links.
The media repository can also be used for out-of-copyright images such as old postcards to compare with the same scene in a current geograph, or for user-generated maps of the area.
You can also annotate other people's images and link them as external images from an article you're writing. To get started with article writing, have a look at this overview: http://www.geograph.org.uk/article/Help_on_formatting_of_articles .