If you're looking for guidelines for your image format, I'd suggest:
JPEG format (not GIF!), image dimensions no bigger than 1024x768, and filesize below 500kB per image.(ideally <200kB)
This should give you decent quality and little to no artifacts when resized to fit the projector. Keep the originals around of course - they may be useful later!
Raw, unsized 2-5MB images (BMP/PSD/TGA, etc) are not ideal for emailing, or displaying on screen!
Most photo/image viewers can do this resizing/saving for you, I can recommend Irfanview (free).