Changing skins

Whenever I go to change skins on Minecraft PE with my favorite skin app, Skinseed, when I want to change my skin I press import and other such that takes me to my photos, I press the desired skin and pops up the slim and larger option however that is when things go wrong. The slim and large options are not the skin I have chosen they are the old skin before I have chosen the new one and the one I press is not the skin I wanted. When I finally get the skin to be perfect and join my little brother on survival I look at his skin. His skin is my current skin. I hope this will get fixed.