I found this Universe: 113 Character Poster, which features a whole bunch of sci-fi characters.
Who is everyone?
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Numbering levels 11–0 from top to bottom, going through each level from left to right (unknown images are in bold).
[a] Waldo (Where's Waldo?)
At the very top of the mountain -- the flag of the Democratic Order Of Planets from Futurama.
Buried on the left side -- Bart Simpson's skull, the idol from Raider's of the Lost Ark.
One of the element stones from Fifth Element -- between the Luggage and the pod racer, laying on the floor.
Kaonashi is holding one of the turrets from Portal.
[c] unknown cylindrical machine or containment (behind the escalator) https://www.imdb.com/title/tt0091341/mediaviewer/rm4276490240
Pepelats — an interplanetary spacecraft (from the Georgian word "pepela" for butterfly) https://en.wikipedia.org/wiki/Kin-dza-dza!
I know giant-brain-in-a-vat screams Mother Brain (Metroid) but the console is absolutely lifted from the X-16 lab in STALKER; the brain vat itself also looks much more like the one from STALKER than any of the Mother Brains, which take up much more space and usually have spikes.
Side point: It's set in the Ukraine and we've got a Ukrainian artist here. The poster behind the Godzilla movie set is another piece of his.
The possible Farscape references don't seem to be accurate with higher-res versions of the picture.
Reddit thread going over the same image here. Given all their reposts, there are probably other similar threads lying around as well. Separate thread here at e621 that someone at Reddit linked with warnings about NSFW ads. (I don't see them, but I use blockers.)
There's also a larger version of the image here. Literally larger: they added a moon, sandworms, and several additional characters into the currently empty spaces.
Level 10 [d] Unknown drawing next to Bowser (hand with an eye?)
Could be cover art from Star Control
Level 7 [c] unknown humanoid boxing Prawn alien