First off, there's the official press kit with most shots in lossless quality (unlike the stuff on the Steam page or on the official site), but what they show is likely very close to the final release. There's also some pre-release gameplay footage.
Earlier shots were made public mostly through the Voidpoint Twitter account. By searching through both Twitter and the IF & Bombshell threads here in the forums (plus some Google search-by-image) I was able to track down the following shots (along some animated GIFs and some other materials), with dates of publication:
- 2016-01-08 https://twitter.com/...525727551320064 https://duke4.net/bo...shots-released/
- 2016-03-12 https://forums.duke4...post__p__241317 https://i.imgur.com/To7gTXn.png
- 2016-03-13 https://forums.duke4...1230__p__241448 https://i.imgur.com/e2XoCtX.png
- 2016-07-02 https://twitter.com/...006554268045312
- 2016-07-09 https://twitter.com/...838533590155264
- 2016-11-20 https://twitter.com/...211740025704448 https://imgur.com/XdLcSGP
- 2016-11-27 https://twitter.com/...646450245025792
- 2016-12-04 https://twitter.com/...218285536956416
- 2016-12-10 https://twitter.com/...519782480117760
- 2017-06-08 https://duke3dmaps.t..._related_post=1
- 2017-05-14 https://twitter.com/...537688837632000
- 2017-05-14 https://twitter.com/...537914147360768
- 2017-07-10 https://twitter.com/...463349278228480
- 2017-11-12 https://twitter.com/...532252953837571
- 2018-03-21 https://forums.duke4...post__p__297456 (not sure if a higher res version of this exists)
- 2018-07-29 https://forums.duke4...post__p__303984 (image uploaded by Fox in response to my question)
- 2018-08-06 https://www.pcgamer....ince-the-1990s/ (concept art)
- 2018-11-25 https://twitter.com/...465600740814848
There may have been other pre-release images exclusive to certain online news publications but I am not aware of any ATM. Also some extra stuff might have been shown only via Discord. I might have missed something while browsing Twitter too, so if you know any other images that should be listed here, please tell!