This is one of those uses which is only for an extremely specific purpose, which is when you are in a bar/pub/restaurant etc. and are placing an order. "Two pints of beer, one lemonade and a packet of nuts, please." "We'll share a bottle of wine, but my daughter would like a lemonade." Even in these situations, some people (including myself) find it more natural to say something else, such as "some lemonade", "a glass of lemonade" etc. That, I think, is the only time when you would use "lemonade" or any other non-branded and non-alcoholic drink as a countable noun. If you're not ordering something (or taking an order) then I would say it's an inappropriate use, so if you're just looking at a photo then a better response would be - to quote you! - "a glass of lemonade". Does that make sense?