As was requested, here's (see attached) the .png image that appears in every folder containing encrypted files in my computer; the image also shows the ransom message. With a little bit of reading, it seems that my misfortune really is consistent with the Cryptowall 4.0 infections. I also learned that there's nothing much I can do about it now so I'm just copying the encrypted files to an external hard drive hoping for a future solution, and trying to recover what can be recovered of the files I lost with a file-recovery software.

Please keep me posted. Much thanks.