All right, after some shopping around for ideas I ended up with Microsoft's OneNote which was not on my radar, given that I'm a Mac user. On a Mac, the app is usable without having to buy the Office suite. (As I understood it, that might not be the case for some Windows operating systems?)
Anyway, long story short, I copy/pasted my notes from Evernote to OneNote and lo and behold - the first image is the one used for the thumbnail! Which is perfect for me as that image is the flower's closeup. I have no idea, if one can actually set another image as thumbnail - apparently the Mac version doesn't have all the goodies the Windows version has - but for my simple purposes it works like a charm, search function included.