He has never been in trouble before, not really
What does "not really" mean? I'd guess that he has done things that would have merited a sanction, but hasn't "got into trouble" with you over. You haven't indicated that he's "in trouble" with you over this incident.
a boy told him to
So says a 5-year-old who is trying to get mummy to take his side against the teacher. Best to clarify this with the teacher, you may find that there was no other boy involved or the boy named was an innocent by-stander/ scape-goat.
If he was doing something another child told him to, you need to teach him not to. He should have told the teacher that X had told him to pull down the girl's pants, not actually done it.
and she told the Teacher
Sounds like you are blaming the girl for getting your child into trouble.
she took his picture off him and put it in the bin.
There may be a missing piece of the story here, e.g. was he told to sit at a table and shouldn't have been drawing, or told to put the picture away and didn't...
He is very upset.
This a distraction to deflect your anger away from him (and it's working, isn't it?)
Says he doesn't want to go to school
He expects you to pander to this. He needs to realise that going to school is non-negotiable, as is not pulling down other children's underwear.
she (teacher) shouts all the time.
Or has told him off firmly more than once? Perhaps he'd better start doing as he is told.
You need to talk calmly to the teacher and work out how you will tackle this together. Be prepared to hear that you haven't got the whole story from your child. Doesn't mean he is lying, just he has only told you the bits he remembers or thinks are relevant.
Good luck - only another 6 years and 180 days of primary school to get through.