What does the "teehee" expression mean?
The head punching and tongue sticking out makes me think the "teehee" is a translation of てへぺろ ("tehepero"), which has a literal meaning of "teehee tongue-sticking-out". It actually has a recent history as a word in its own right: this blog post and this crunchyroll news article have some information about it. 

Basically, it's the word that goes with teehee+punching head+tongue sticking out, and the meaning is primarily focused on definition 3) that Omega posted - it sweeps under the rug a mistake or ignorance of the speaker with a playful gesture. A Japanese source for this meaning is this scan of an article about tehepero being Japanese neologism of the year, although the expression is likely related to peko-chan (see What is the origin of this facial expression with one&#39;s tongue sticking out?).