Yeah, that would be my first reaction, too.
I came across this artwork while browsing through my archives and thought I'd post it so it could see the light of day. While it's a lovely depiction of Dagwood delivering some parental discipline, I don't think it's a panel from the actual comic strip. Compare it to another picture of daughter Cookie getting a good "talking to" from her father.
This is an actual scene that appeared in the comic strip. Compare it to the first picture. While the drawing of Dagwood is good, you can tell that it's a little bit off. And Cookie is just all wrong, from the clothing to the face--even the way the hair is drawn. Add to that the fact that her bottom is so prominently featured in the first drawing--an everyday family comic strip wouldn't be quite that blatant.
So while I certainly enjoy the first drawing, I have to conclude that it's a piece of fan art, and not some long lost Blondie panel from ages ago. It's a pity there's nothing to identify the artist, so let's just send a "congratulations on a job well done" out into the Universe and hope it somehow reaches the right person.