That depends on the opinions of the local prosecutor and judicial system in the area where you live. Generally it would not be considered porn, but if the context where the picture was posted gives a reasonable suspicion that it is intended to get someone excited, then it would be porn. Understand that if something gets a person excited, it could be porn to them; some people get excited about shoes, so pictures of shoes could be porn for bokep chindo them whereas for most people it would just be another piece of junk mail or a sales advertisement.
When you stop and consider it for a moment however, giving a wedgie is usually an effort to embarrass, intimidate, or humiliate another person. Even if it is only intended to get laughs, it is still a mean thing to do. Posting pictures of it amplifies its meanness.
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