Saskia, WWI research can be a little on the frustrating side at times. You can have a decent amount of information and still get nowhere unfortunately. It really is pot luck.
I've found little things of interest which can be helpful/interesting.
For example: A lot of men, enlisted but never went anywhere near the front which again makes them difficult to track. Instead they were assigned to Labour Companies who were tasked with jobs such as digging trenches, maintaining the railways and supply lines etc. Very very few records relating to the Labour Coys were ever made. They were often older men who were of a 'lesser' quality (eg over 35). There was also a lot of prejudice towards Irishmen who enlisted to English units and they were often assigned to these tasks too (to put it bluntly, they weren't trusted with a gun). Despite not going over the top, these men were still in danger and were often killed (accidents, shells, gas for example).
Miners are also an interesting case. They were regarded as key workers because of their civilian trade. Many were kept in reserve even when they enlisted and continued to work to supply the war machine. Or they were assigned to Labour Coys for specialist digging skills and never carried a gun.
So any little details you might think are unimportant or not relevant can help you understand someone's war, even if you can't find more detailed records for them.
Photos are generally pretty hard to come by - they seem to be more easily found for men who enlisted earlier in the war or through private collections rather than being publicly available. Regimental HQ may have some, but from experience what exists tends to be of officers, large group shots without names and there is little recorded for individual soliders unless they are someone of note. You usually have to visit them personally often at a fee - getting someone else to do it can be an option but this costs extra and the chances are more likely that you will only find general information rather than something that names your relative.
The National Archives is still in the process of uploading war diaries. A lot are now up, but its not due to be completed until later this year. If you are looking for a diary and can't find it, it may be there in a couple of months.
I also find trying to google for personal accounts from other men in the same battalion a useful exercise. It can be a waste of time, but sometimes you get lucky.
Below are a couple of resources which may also be able to help you with this:
www.europeana1914-1918.eu/en Are trying to collect stories of individual men to preserve for the future. Imo its not as good currently as its promise, but there may be things of interest on there.
The old BBC remembrance wall is also useful. Its an utter pain to search via google as its been superseded by more recent WWI resources, but the personal accounts by regiment can be a useful incite. www.bbc.co.uk/remembrance/wall/