A Voyage Into Yesterday
I assume, from the tagline, that the monster on the cover is supposed to be some kind of dinosaur, but it sure looks like a dragon to me.
Later published as a paperback
This painting sold for over $10,000
The Dwellers In The Mirage has been featured here several times before.
The cover is by Lawrence Sterne Stevens, interior illustrations are by Virgil Finlay
This story can be read in full at Project Gutenberg and scans of the actual pages of this issue are here.