According to that font of all knowledge known as Wikipedia, Frank Frazetta's first confirmed pencil & ink work in the funny books appeared in Treasure Comics #7 (June-July 1946). Here it is. Don't look a lot like him, but hey, he was only 18 at the time. I guess we should cut him some slack. JonTheScanner stuck it up on comicbookplus.
Frederick Nebel's best work appeared in the pages of Black Mask and Dime Detective. You can pick up any of the nine Altus Press volumes featuring MacBride & Kennedy, Cardigan, and Tough Dick Donahue, and be amazed at his command of hardboiled prose. He was one of the top three best writers for those magazines, and you know who the other two are.
But prior to - and while - writing for those detective mags, he turned out a lot of air adventure stories set in the South Seas, a somewhat smaller - but still significant - chunk of Northwest stories, and a lesser number of South Seas adventures without the airplanes. And it was all good stuff. I can't say it rises to the level of his detective work, but it was certainly a cut or two above most pulp fiction in those genres.
A few years back, Tom Roberts put out these six collections featuring some of those high-flying, cold climate and warm climate adventure stories, and all are well worth your notice. The Black Dog Books site is not accepting orders at the moment, but they're all available as Print On Demand from Amazon. (I had a little involvement with a couple of them myself, but don't let that stop you!)
Altus Press has also done three volumes (so far) covering similar territory, and they're great too.
In case you missed it, yesterday I posted pics of Black Dog's LESTER DENT LIBRARY (HERE)