I have access to my iPhone's file system.

As stated in this answer by me, it is a confirmed bug where skins, when in the photo library, are converted straight into .jpeg, instead of the required .png format.

How do I change my skin?

To simply put it: Which app folder directory do I upload the skin file into?

Also: No, replacing the char.png located in the app package IS NOT THE ANSWER, since nobody else can see that skin, except for you, as it's changing the default skin, rather than the player's custom skin, and no, using the in-game skin changer is not an answer as, in the first place, it does not work!

Note: This may have already been answered in a totally unrelated question somewhere. I just can't seem to find it.