The most forgiving reading of the intent of this sort of agreement is that "it is the basic function of a web browser to transfer your stuff across the internet, and it is the basic function of a website to do stuff like make thumbnails of your images and send them to people looking at your stuff, and you are cool with us doing this" but it always ends up being written in the most incredibly grabby way possible, demanding a perpetual, irrevocable license for all potential future uses so that you can't sue if one thumbnail gets forgotten when you delete your stuff or because it got turned into a new format or something.
And this kind of license just happens to cover other stuff nobody thought of at the time like "we can totally train an AI on everything you generate and let people ask it to generate work explicitly derivative of your work without owing you a single cent". Which is a total dick move. So's stuff like "track everything you do and share that data with a distributed surveillance industry that sprung up around advertising". And eventually some asshole comes along and says "hey I could make a lot of money doing this thing that everyone "agreed" to when they scrolled down that lengthy terms of service and hit OK".