Regimental Museums may have pictures of your relatives, but they won't necessarily be named or indexed online, so ideally you would need to know which battalion, or better still, which company your GG Grandfather and Uncle served in, and hope that there are some group photos.
Since both men died during the war, they may well have had brief obituaries perhaps accompanied by photos, in the local newspaper(s). Another place to look is in Rolls of Honour in local churches or community centres, places of work etc, although these are less likely to have photos.
Of course the most likely place to find such photos is within the family, as many men had portraits done before leaving for the war, to give to parents, siblings or wives. But I assume that you have already explored this avenue of research.
