Hi. You can check what exactly is taking space by using the “select Data files to delete” button in your “My Foundry” page as it will list every world, module and system with their sizes.
If a single world is too large, then that might have been a misbehaving module causing it. I’d suggest looking over this tutorial : World Size Check
If you installed a module from within Foundry, it would still have used the Bazaar, so that shouldn’t have been a problem. Unless it was a premium module (one where you had to add the content key into your account on foundryvtt.com) in which case, read the “optimizing data usage” from here : Installing/updating Licensed Premium modules
Finally, yes, embedding images inside journal entries could be embedding the entire image inside the database, which would be pretty bad in general. I think a module makes it do that, and that can cause the game database to grow fast and cause the game to load slower as well as a result. That first link I gave should be a good help in finding if that’s the problem.
I hope this helps point you in the right direction, and if there’s more info you need, don’t hesitate to ask.