It sounds like it’s possibly a game that is not correctly tagged. Are you able to point me to where you saw it if you are unable to go to the page to submit a report. You should never see any explicit content when browsing for games unless you have removed content filters.

Regarding your question directly, in our content guidelines we ask that cover images are not NSFW. But, as of this post, assuming the page is properly classified to not show up in regular categories, we actively aren’t enforcing this restriction. (Although we recommend devs to follow our policies at all times in case we do start more strictly enforcing that rule)

Make sure you have not checked that box in your settings, assuming it were for horror games.

Content — How content on itch.io is shown to you

• Show content marked as adult in search & browse

Also make sure you do not use a tag like "adult", for the same reason (seeing it on a popular horror game you visited by external link). That ignores the setting.

If you browsed any of the popular sections chances are, someone else will report it for being miscategorized rather quickly. It is the popular sections after all and many people see those.

And if you or that someone else opens a game link in a new private window (to clear cookies in that new window) correctly as nsfw marked games will display a warning message. Of course if the thumbnail is already nsfw, you might not want to do that. Also because you will get that hover preview with screenshots. Even if the cover image of a nsfw game might be sfw-ish, the screenshots probably are not.