I'm not too familiar with the gallery feature, but taking a quick look, it appears you cannot "like" a photo when you access it through the individual gallery forum, but you can if you go through the Latest Gallery Posts button on the main page. Maybe someone more familiar with the feature can add some clarification.
Hi whippet. As far as I can determine you can only give a 'like' through perusing the latest gallery posts as a whole rather than an individual member's gallery. You will find the 'like' button over to the far right hand side next to the quote button.