Currently assisting the wife with documents for the UK visitor visa. Someone in my family will be her sponsor and will give all the relevant documents.

 

The wife is insisting that I need to write a letter too, (so 3 letters in total), which provides the past 6 months of my (two) Thai bank accounts, plus photos of us together.

 

I don't want to overload them with documents so checking its a requirement? (both herself and the sponsor will be providing bank statements).

Your wife’s advice is correct, a short covering letter from you may help convince the decision maker of her ties here.

1 hour ago, decline said:

 

I don't want to overload them with documents

Forget the photos, they don't want them (and specifically say so).