RAK means Random Act of Kindness. Basically, you are sending a card, gift, ribbon, images, etc to another person (doesn't have to be a stamper) for sympathy, birthday, etc, or just for no reason.
That happens a lot around here and it's really fun. I enjoy sending RAK's just as much as receiving them.
As for your images you're looking for, check out the swaps section for images or products. There you can post an ISO thread or even find someone who has the set you're looking for posted and swap with them. Check this thread out:
Forums at Splitcoaststampers I've participated in this image swap many times and it's great. As for the other swaps, each one has listed in the swap directions the requirements. Any further questions about the swap can be directed to the swaps hostess. HTH