Hmmm. You probably need to send them as Jpegs and you need to remember that pictures that look good on the web, often look bad when printed - if the filesize is too small.
For a 4x6 inch standard picture you'll probably want that file to be about 1.5 megs or more.
This means that for your photos that you want to print, take them at the HIGHEST resolution that your camera will allow. Then if you use a graphics program to work on them, save the file at the highest Jpeg quality that your image program will allow.
Hope that helps.