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Thanks Kristina, Great - thanks for clarifying, that's the exact explanation I was looking for. Good news for a wide collection of URLs Best , Jon

Having keywords in your image filenames is a good idea, but it doesn't harm it to add some dynamically generated strings at the end if that's necessary for technical reasons. As one example, I maintain a site which has user profile pictures, and by adding a dynamic string to the end of profile pictures it prevents the situation where the user uploads a new profile picture but see's the old one because of their browser cache. Definitely use relevant text in your alt and title attributes, but don't sweat a few extra characters added on a filename if it will help your developers.