From the page you linked.
Here’s the rub. That definition isn’t clear in the slightest.
There are several streams on ImgFlip reserved for where “sexy” images may be posted if marked NSFW. Some would call that “porn” pejoratively, others would call it erotic art, others would say it’s just photography.
There’s a more robust definition somewhere of what ImgFlip considers to be TOS-violating “porn,” but even that definition is not perfect.
I had a discussion with a couple site mods where I tried to hammer it out the other day, and we got even farther than the official printed definition, but not totally all the way there — since I posted content that complied with their guidance and still had it deleted.
Here’s the thread where we talked about it, warning NSFW content ahead.
imgflip.com/i/4dilxd
In the end, it’s still a gray area. I guess it just depends on who’s looking at it that day.