Screenshot also needs to be 256 x 256 pixels and Index Colored.
Also in Level properties you can add mapname, playercount and other info. And as usual i forgot which one it is, but if you add text to a certain entry than the screenshot wont show up and this differs from UT and Unreal Gold.
So if you added info to all entries and the screenshot doesnt show up, remove all entries and check if that works.
If so re-add all entries untill the screenshot disapears again. In that case leave that entry blanc. So oddly enough the entries can influence the apearence of the screenshot, and as said, it differs between UT and Unreal Gold.
And you also should make sure to save the screenshot with the appropriate Indexing. If that is wrong import simply might fail. You can find the correct indexing here:
Bottom of the page
Also, i am not sure but it could also be you have to import is as screenshot rather than Screenshot (hence the capital S).
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