Photos are a vital selling tool that should be used in all your charity's publications.
Including photos of your charity's projects (either action shots or the results of your work) are more evocative than a text explanation. Photos of staff will make the charity seem more approachable.
It is well worth investing in professional photography to get a bank of good images that you can use in different marketing materials. If you cannot take photos of beneficiaries to protect their anonymity, consider using stock photography from image libraries.
A good stock photography site that offers free images is stock.xchng. The images are free of charge, but the site asks that you stick to certain rules and for some photos you need to notify and credit the photographer. To avoid the hassle of notifying or crediting photographers, limit your search to photos that are not restricted (by selecting No in the 'Restricted Ok' drop down box on the advanced search page).