Unbeknownst to 16 year old Andrew or Dani the tattoos are made of an ink that contains a special oil that absorbs into the skin and activates a very powerful transformation that generally completes in only 12 hours or less. The tattoos come in a multitude of images though the actual picture means nothing; when the ink reaches the bloodstream it responds to the perception the user had towards the image, therefore the end result is always what the user expected it to be even if the image was damaged upon application.
This also means that the results of the same tattoo can have wildly varying implications; which are based on the users expectations of the effect they will pull off. For example, a child with a star tattoo, imagining that it makes them a superhero, will become what they expect a superhero to be (within the bodies limits). If the child puts it on their father, lets hope the fathers perception is more akin to, "this makes me look like a loving dad" as opposed to "this makes me look stupid." Better yet, if he roleplays with his child, he will end up looking like a superhero.
The chemical interactions which create the transformation are set within the first half hour to hour of application. This means, do not apply immediately before sleeping as it will have no effect or possibly unexpected side effects. The slower heart rate while sleeping prolongs the time needed to set the target transformation, meaning the chemicals may still be volatile while entering the first or even second dream of the night.
The effects, both mental and physical, are usually permanent depending on the users expectation. Some situations may cause temporary effects, like if someone expected a tattoo to get them high. That person would be high for 11 hours or so after the expectation has set.
For significant physical changes the transformational oil usually takes several hours with minor effects and then changes occur rapidly.