Because of other people doing these things and turning out to be liars, everyone just assumes and jumps to conclusions.
Nobody is jumping to any conclusions. Either way you play this, you're a liar.
The picture could be legit and actually be you and you're claiming it's fake, in which case you're a liar.
The picture could be fake and you posted it claiming to be you, only revealing the truth after being scolded, in which case you're a liar.
You're excuses strike me as being a little too convenient, so my bet is that you're just ugly and trying to cover your ass.