Galaxy Science Fiction, Fantastic Universe, and If: Worlds of Science Fiction. Are all really high quality. The pictures have good craftsmanship, they're creative, and each image really seems to be telling a story. Amazing Stories is worth checking out just for the sheer volume and if you want that super pulp style art, Planet Stories has you covered.
Just a quick FYI, there is stuff in there that might be considered NSFW, so be sure to click at your own risk.
Compared to a lot of the images in those galleries, the one I linked to is pretty tame. There's so much to love about it though, I don't even know where to start. For one, I just love the overall shape of the ship. It looks sleek and fast and nimble yet at the same time, it looks so aggressive and powerful, from the fins shaped like blades to the massive rockets. Speaking of massive, look at the three men against the hull to give this ship a sense of scale. The whole thing is huge. Lastly, there's not a ton of background to distract you, the two men in the foreground add a sense of motion and drama to the whole thing, but this painting is about the ship, front and center and just fucking awesome.
I kind of want to see if I can find this issue, if only to have the cover.