Misakas here to remind you to conserve your bullets and use your ammo wisely, and makes Engineers short circuit.
The text is a play on what she says in the To aru Majutsu no Index-tan specials "Conserve Electricity and be Power Smart"
The original image is from: http://konachan.com/post/show/63039/misaka_mikoto-to_aru_kagaku_no_railgun-transparent
Update 1: Blu version now included.
Update 2: Different & better Blu logo used