Both of these 1963 covers are by Jack Kirby and Dick Ayers.

The stories they promote were drawn by Joe Sinnott and written by Robert Bernstein.

   I was a Bernstein fan as a child even though I didn't know his name. The barbershop where I went as a child had DCs, mostly Superman titles. Many of the stories were by Bernstein including this 1961 Swan-drawn one.

 

"I guessed that Loki must be the original ancestor of Mr. Mxyzptlk and that he, too, can be sent to the 5th dimension for 90 days if he could be tricked into saying his name backwards." JIMMY OLSEN #55 (Jimmy the Red, Thor's Best Pal)