Hi Hazey
My DD has only a handful of words but she too is very well able to communicate what she wants by a combination of signing, gesture and vocalisation (could also be called screeching/crying/shouting
).
She was very slow to start signing - we started at about 18 months and it took probably a year plus before we consistently had "more" and "all gone". We then built up from there (and her ability to learn new signs got quicker and quicker).
We really just focused on "requesting" signs to start with - so as Dev said up thread, identifying the things she really wanted and then keeping them out of reach so that she had to ask each time she wanted something. So, typically, this was things to eat or to play with or "Mummy" or "Daddy". Because she was motivated by those items, she learnt pretty quickly what she had to do to get them!
What worked for us was tackling 2 or 3 signs at a time and focusing on those until they were mastered and then adding others. By mastered I mean an approximation of the sign done proactively. My DD's fine motor skills limit the accuracy of her signing too which is why we've always accepted approximations.
We're now at the stage where she's starting to use signs to label things as well as to request things she wants e.g. we show her a picture and say "what is it" and she answers with the sign.
I was told that signing uses the same part of the brain as speaking and so that is why our consultant was keen that we focus on it. That said, she still has no words 
As for showing the picture at the same time, I don't feel qualified to comment on that. All I will say is that we dropped using PECs in favour of focusing on sign and in hindsight that may have been a mistake. Now, we're at the stage where she doesn't have the manual dexterity to differentiate lots of different signs and her signing vocabulary isn't wide enough for the breadth of what she wants to communicate - so we're back to introducing PECs in conjunction with sign as a prelude to getting some kind of talker device.
HTH a little and doesn't add to your confusion 
p.s. ref your other thread my DD has the mosaic form of a v rare chromosome disorder (no name). We were told this should mean that the effect of the chromosome abnormality is somewhat diluted but how you can ever measure that I don't know. Good luck with getting your diagnosis. x