Photographs of soldiers writing letters are quite common among official photos of the First World War-- it was in the interests of officials to reassure those at home that it wasn't so bad, after all, and seeing the boys on the front writing and reading letters from home was tremendously reassuring. Soldiers did, of course, write plenty of letters...even if the worst parts were usually omitted from correspondence, by individual choice as well as by official censorship. I find these types of photographs very moving, knowing just how much it must have meant to the poor men at the front to hear from home, and to those at home to know their loved ones were still all right, for at least another day.
All but the last are from the World War One Official Photographs collection of the National Library of Scotland.
Officer writing on sandbags. Source
Gas sentry writing a postcard. Source
Soldiers with letters in a captured German dugout. Source
Gas sentry. Source
Scottish soldier writing on a staircase to nowhere. Source
Portuguese soldiers writing. Source
Letters in the trenches. From the Dutch National Archives. Source