Does anyone have any experience with this?
I recently lost a friend and found a photo of Williams Shatner as Captain Kirk, signed, amongst his paperwork.
Can’t find any letter of authenticity and from a quick Google looks like there is probably some scam websites who offer their opinion and “certificates”
Not sure if it’s worth trying to authenticate etc. Anyone got any ideas?
First of all I’m sorry for your loss… 
I know police has experts that use existing written documents to try and build a “profile” of someone’s handwriting and use it to verify whether a particular document is likely to have been written by that person, so you should be able to get in touch with such an expert and have them do the same work for you - I’d expect their time to be quite expensive though.
I’m sorry for your loss 
, and as for if it’s worth trying to authenticate - how much is it worth? I imagine there are a lot of signed works, and likely not going to be worth the cost of hiring an expert to authenticate.
A better option may be to look up his signature and ensure it looks like other signed works convincingly enough to make you happy 
Considering Shatner is still on the convention circuit and, at least his appearance at Destination Star Trek Germany (April 2018), is giving autographs for 75 EUR: the signed photo is extremely unlikely to be worth more than that: you could get a “newly signed with photographic evidence of him signing” for that price.
I might just place it in storage for now, then.