Let's talk about gear first. You'll need a small, portable notebook to keep all your images and written prompts in.
Oblique Strategies is a deck of cards first published by Brian Eno that contained a cryptic quote, question or remark on them to get your mind thinking.
Once you've read the words on the card(s): follow the direction on the card or write down your thoughts and reactions to the images on the card and what they mean to you. It's that simple.
It was created as a way to quickly generate new ideas when your mind goes blank. While Oblique Strategies decks are hard to obtain these days, you can use this concept and the Portable Muse idea to hack together your own Oblique Strategies deck.