You can put links (URL or URI) to web pages and geograph photos into your description, and this will display with the text "link" in the description. Always leave a space after the link to avoid punctuation marks being included in the link. A link to any web resource will work, including a link to a photo on geograph such as http://www.geograph.org.uk/photo/12345. Note that the set of numbers at the end of the url is the photo ID.
You can also link directly to a pic on geograph by entering the ID surrounded by 2 or 3 square brackets e.g. [[12345]] This will display as a link to (e.g.) " NN2544 : Coire Toaig".
An easy way of getting ID's for your recent photo's is to click on "recent uploads", where the id's are listed in square brackets for you to copy. This can be helpful when creating links before pics are moderated.
For more detail, and for editing pics that have already been moderated, we suggest taking a look at Rudi Winter's tutorial (click on "More information on this topic... " below)