The way the messages are visually portrayed. A picture book is like studying a cartoon, except you have a whole lot of them to analyse at the same time.
Eg, are the "images" of the protagonist(s) in the book realistic? If they are, the illustrator deliberately drew them this way for the audience to empathise and connect better with them - thus the audience will sympathise and support the views/ideals of the protagonist (what the composer wants). Conversely, are the "bad guys" (if any) portrayed in a negative light (eg unnatural features, dark colours, etc.) so that the audience doesn't like them - therefore will not support their views/ideals (again, what the composer wants).
What you look for also depends on what aspect of the book you're focusing on. I'm assuming that you'll be spending most of your time analysing the actual illustrations and not the text/plot. Therefore, you would look at the techniques associated with pictures and visual stimuli (eg, background, foreground, colour, text, positioning, etc.)
By taking a picture, you're approaching "image" in a literal sense - ie, you're looking at the actual image an illustrator creates. Do not forget - and DO NOT EVER write - that an image is only created visually.
For a picture book, you have to link the picture to the message the composer is trying to convey and HOW, through the pictures, this message is conveyed. A message is usually conveyed through the image by looking at how the image affects the audience (eg, do we like/dislike the image of the characters, what is portrayed as being important through positioning in the foreground, etc).