The screenshot is available but it's about the same size as Speed Dials, they're not extra big like Stash used to be.

I don't know if the importation from Stash to bookmarks will keep your current screenshots or replace them with new ones.

I suggest you to keep a copy of your whole profile so you can try it out and if you don't like it you can rollback to at least Opera 24 (I think it's the last version which still has Stash).
From there on you'd have to decide what to do with your data... Grab an extension or start using a website that mimics the functionality for you... Or something that allows you to update the browser so you're not affected by security and other kinds of issues. You really shouldn't have hold to Opera 15 for that long...