Hi Marilyn --
Are you using Aperture? If so, you have come to the right place. Welcome to our sun-lit clearing in the orchard .
We need some more information to help you. What version of Aperture? Is 10.7.5 correct for your OS version? Have you run Software update recently?
What camera? Are you doing anything unusual during import or after?
Does it happen with _any_ Image based on a RAW Original?
Does it happen regardless of which Image Export Preset you use? (Try TIFF as well as JPG, "original size", make sure the quality slider for JPG is set to at least 8. You can Quick View the exported files from Finder, or open them in Preview.)
The other thing to check is the Image Size setting in the box that will be seen beneath the draft message when an image is added to a message in te Compose window. If ever that was set to anything other than Actual Size, that setting will become the default until changed again. Check that setting.
Are you shooting in (recommended) RAW+JPEG? RAW files from the camera have no sharpening or other adjustments applied to them by the camera, while the JPEGs do. So unedited RAW files will usually look less good than in-camera JPEGs. My guess is that the pix you see that you like may be camera JPEGs and the ones you do not like are unedited RAW files.
There are various settings and workflow techniques within Aperture that determine which file is used where.
What color space are you using in your camera? Most are set to sAdobe, which will use a narrow group of colors. sAdobe is for putting images on the web, which is why they look good on yoru monitor, but not so much when you print. When you print you need your printer to output a larger gamut of colors. The result is you will get washed looking photos when you print. So Adobe 1998 is the selection you should make in your camera if you intend on printing.
Also, keep in mind that RAW means raw, in that nothing is done to the file. You should make any necessary adjustments to exposure, color tone and sharpening before printing.