1. Writing about the terminal cuteness of your dog or grandchild will make a book that others want to read.
2. Anyone can write a picture book.
3. You can make a picture book starring an inanimate object as long as you use alliteration. I remember one very nice person (a writer for adults) who enthused to me about her manuscript WILLAMINA THE WAVE. (I've changed some of the details so this person won't be embarrassed if s/he happens to read this.)
4. Story ideas about an animal who can't do the thing that all others of his species can do, but learns to feel special anyway: The dog who can't bark, the fish who can't swim, the bird who can't fly, etc. etc.
5. Stories based on something that happened in the news--yesterday.
6. You know in your heart that the rules don't apply to you, so you don't need to bother to learn them. If the guidelines say your story should be between 300 and 600 words long, that you should research who publishes what, and your ms. should be properly formatted, there's nothing wrong with sending a 3000 word, single-spaced picture book, formatted however you think it should look to any old editor.
7. If you think it's funny, a four-year-old will think it's funny too.
8. Doing something extra to make your manuscript stand out: Sending glitter, putting cute stickers all over it, typing it up in colorful font, etc.
Now having said all that (and you may not want to tell them this, but it's the truth)...
I've published manuscripts that have broken every single one of these rules.
1. One of my illustrators put my dog in one of my (most successful) books.
2. My first picture book was acquired by the first publisher I sent it out to. Did I know what I was doing? Absolutely not.
3 and 4. I'm the author of BORIS THE BORING BOAR. (Another good rule is: Don't put the word "boring" in the title of your manuscript.) This is the story of a wolf who becomes a vegetarian.
5. I'm the author of SOMETIMES BAD THINGS HAPPEN, a picture book I sold the day after 9/11.
6. I've sold really, really long picture book manuscripts.
7. I've sold manuscripts that were funny, but mostly to adults.
8. I sent a story about a witch in a black envelope.
So the last misconception about picture books is:
9. Ninety-nine percent of the time, you should follow all the rules. They really will help you not get eliminated. BUT you won't get published just because you follow the rules. And sometimes you'll get published even though you don't. (But don't count on it.)