How to politely tell the PI I'm working with to use the vector format for images
From the comments I gather that you are afraid to criticise your PI for producing low quality images, which is natural.

I think that a good course of action here would be an e-mail where you tell your PI that your journal requires vector based images. Then you say that you know that he is a busy man and that you are willing to reproduce his images in Illustrator if that is okay with him (assuming the pictures are easily reproducable). That way he will probably thank you for going the extra mile and taking work off his shoulders.

I would refrain from telling him to make better pictures himself, because he has probably done it the same way for decades and it is hard to change such habits. 

Good luck with your publication!