I agree with you in all your comments, except this one point, which arguably is at the center of discussion.

And beyond all of this, why should my personal blog be regulated by the same laws as encyclopaedias, books and magazines? It's just my tiny hub, after all, where in any other site I'd just post collages and memes without thinking that I may be "violating" an author's rights (harsh word, ain't it? For a personal blog...)

I think as @maverickinvictus said, it doesn't matter if it's your personal blog or not. Monetization is where the line is. If you are making a lot of money using a copyrighted image without obtaining a license, then it will be a problem. Same rules as a commercial blogger. I don't think you can claim this is just a small personal corner of the internet and be exempt from the same rules as a professional blogger.

That being said, a lot of what you are saying is true-- there are certain rules that don't really make any sense, and it revolves around the "fair use clause" which I don't really know enough about and have been scratching my head around because for example: can I use a movie trailer poster? To me this should be an obvious yes, but there seem to be some set rules about sizes and such (too lazy to look up now, but there's a section in Wikipedia that tries to explain the rules here, and Wikipedia does not even have the same rules as a professional blogger due to its free nature)

Finally... For a source that you cannot properly attribute, I'm with @bogenschleif , I would document that it is unknown and that you tried the Google search with no good results, etc. I don't think you should be banned from using it. Also hilariously there's some sort of fair use loophole where you make derivative work or something... Though I don't know exactly how that works.