This Flickr group is a pool of images for special events, sermon series and other items at churches. Browse the images for ideas to spice up your marketing material or website. But also read the comments for the community's critique of the images and maybe even leave your own input. If you have some images you are working on, upload them and you'll get feedback on what's good and what needs to change. Also this site is a part of Church Marketing Sucks.
While the images are copyrighted, if you find some that you could use contact the person you uploaded them and see if they'll grant you permission to use them.