I'm looking mainly for pictures (preferably the more unknown ones), poems written in memory of soldiers, letters written to such soldiers and every other material than can transform the "cold distant past" into a more "warm and humane" one. This is for a personnel project, and while not directly about history it is related enough that asking a historian would make sense.
There are a lot of sites which might be of interest to you. For the Anglosphere specifically, it is hard to beat the Imperial War Museums which, as you might expect, have massive holdings for WWI. You can check them out at https://www.iwm.org.uk.
Another great resource is Europeana.eu, which is run by the European Union, and provides access to collections from institutions throughout Europe. They have a specific portal for material in the 1914-1918 window, including photos, letters, art, and so on. You can check that out at https://www.europeana.eu/portal/en/collections/world-war-I
For the US, I always like to plug the Veterans History Project, as they are a great program done through the Library of Congress, and have done AMAs with us in the past as well. WWI isn't their largest collection, but there is definitely material you can find there which will be of interest. Head over to https://memory.loc.gov/diglib/vhp/html/search/search.html. I found 422 hits searching for WWI, although they won't all necessarily be digitized.