Advice to new authors
When submitting a picture book to a publisher you will not need to commission an illustrator. This is the sole responsibility of the publisher should they go ahead and contract you to publish your book. The publishing team will select a suitable illustrator and will handle the briefing of the illustrator and payments etc. It will be a waste of your money and time to get it illustrated before submitting it as you will not know the publishers preferred format for the book either.
However, if you are illustrating your book yourself then you will be required to present a 'dummy' of the book with rough illustrations and text on each page and some character roughs and one final piece of art to show technique. It will not be sensible to illustrate the entire book at this stage as you will not know the format the publisher will select for your book. Most picture books are 32 pages in length including the title page and imprint page.
For self publishing you will need a designer to format your book and to brief the illustrator. You will then have the expense of an illustrator, a printer and marketing and distribution person. Or you may wish to check out the following link from the Society of Authors website to help you.
If you are contacting the Illustration agency for a quote for an illustrator if you are self publishing, please make sure you have an idea of how many pages and pictures you require. We cannot give a firm quote until we see a layout and the manuscript.