Take good quality photographs preferably on a decent digital camera (anything over 3 megapixels should be fine), and make sure you process them correctly for the web - if you have a budding photographer in your congregation enlist their help!
Flash photography whilst sometimes necessary comes with problems such as red eye, un-natural skin tones, odd shadows and de-saturation of colour - avoid where possible.
Keep the style and placing of your pictures consistent in your web pages.
Remember that the pictures should be relevant to the content of the page.