When you read a picture book, you might find one or two sentences per page. Assuming you are not also the illustrator (in which case you submit the pictures with the words on them), what would your manuscript look like? Do you just type up the whole story in regular format and submit it (even if it’s only 2 pages long then) or do you submit it as you imagine it (say, 30 pages, each with one sentence on it). And do you suggest illustrations?
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This is a really good question, because many people assume we want art with manuscripts. In fact we want to judge the ms on its own merits before thinking about the art. This means don’t send in pictures by a friend/relative who you think would be perfect as an illustrator; don’t apologize for not being able to draw yourself; and as much as possible resist the temptation to provided suggestions for art. You just submit the text running continuously as a story. If you have strong feelings on the subject, you can indicate where you want the page breaks to fall (remembering that a picture book has 32 pages and 2-4 of those will be taken up with front matter).
If an art note is necessary for the text to make sense because the text doesn’t make clear what is actually happening (e.g. if the text “Bruno is a good dog” needs to show Bruno peeing on the carpet) keep the illustration note as brief as possible. Picture book artists are fantastically creative, imaginative talents (when we’re lucky), and contribute as much to the making of the book as the author. We like to leave the illustrators free to play with whatever ideas they like. And the illustrations will probably be wildly different, funnier, and richer than what you’re picturing.