I love that I taught kinder before teaching 7/8 grade! I use picture books all the time to introduce topics, make connections, and start discussions that lead to more complex thinking. We began discussing symbolism in my 8th grade LA class - we talked about the flag, used the text, they came up with some of their own. Today I read Mrs. Spitzer's Garden and before the end, I had them write down if they recognized any symbolism or not. I read what they wrote out loud, read the ending, and had lots of ooooohhhhhhsss!!!! Wonderful immediate assessment of who got it and who didn't! Easy for them to understand and now we can move on to more difficult pieces!
For anyone teaching history Dr. Seuss' The Butter Battle Book is incredible. I use it at the beginning of the year to help introduce the concept that history is everywhere. I'd love to see what other people use in their subject because I too share a love of picture books.
I love: Paul Fleischmann's Weslandia to introduce the concept of civilization. Ludmilla Zeman's Gilgamesh during Mesopotamia. Aliki's A Medieval Feast when I teach about the manor system in Medieval Europe.
John, Paul, George, and Ben by Lane Smith is great for US Hist. I used Anansi the Spider, Why Do Mosquitos Buzz in People's Ears, and Who's In Rabbit's House? when we studied African Kingdoms. I use fairy tales all the time in LA - elements of plot, point of view, character traits, theme, setting, etc.
The WritingFix website has a collection of lessons that go along with 60+ picture book titles. To find it quickly, type Writing Fix Picture Books into Google!