Is it obligatory to lower one's gaze from a cartoon poster of fictional characters—sea creatures playing music—and is looking at it considered witnessing a forbidden act? Furthermore, is it obligatory to close the room door so that no one sees it?

It is not possible to definitively rule these images as forbidden, due to the possibility of a reason for permission, such as unclear features or the absence of what life cannot be sustained without. Assuming they are forbidden, the prohibition relates to whoever drew or hung them, not merely to looking at them. And whoever views them does not fall under the threat mentioned for image-makers.