Im stuck here too, the only thing Ive found so far is a 34$ pluggin which ... wtf ?? There is no way I can keep going like this ! I share documentation on my website, I have like 10 screenshots for each pages, having no folder is making a huge mess, even after renaming each picture. 

If there is no free option, I'll check another provider and leave odoo for good. This is a basic feature available everywhere else...

edit : the suggestion was made 2y ago and wasn't considered nor even acknowledged by the support in all this time. Can't share link yet but search for "Suggestion for image management for the website, mailings..."

a non existant support, to that point, is terrifying IMO