While true that you cannot know whether the client will request the images, you can use their user agent to make a pretty decent prediction. There will be corn

While true that you cannot know whether the client will request the images, you can use their user agent to make a pretty decent prediction. There will be corner cases where you are wrong, but most of the time your prediction will be true. For instance, if a bot is pretending to be a Chrome browser, you'd think it was a regular client, but in fact it was not. But that's the bot's fault, not your implementation.