When you are saving your pictures for websites or emails, you should definitely compress the images unless you want them to make prints. If you don’t compress the prints, it could take over 10 minutes to download a single 5 Megapixel photo.
This tip will focus on using iPhoto and Photoshop Elements to compress pictures.
iPhoto:
1. Export the picture
If you are using iPhoto 5, bundled with iLife 2005, you can export from the Share menu and then click export. If you using an older version of iPhoto, click on the File menu and then Export.

2. Choose the proper size
I compress all the photos I email to about 800x600, you can choose smaller resolutions for dialup users. If I post them to the website I usually compress them down to 1024x768.

Photoshop Elements:
1. Open the photo in Photoshop Elements
Open the original photograph in Photoshop Elements.
2. Scale down the image
Choose a resolution that you like, I use 800x600 for emails and 1024x768 for websites. This can be done from the Image Menu, Resize, and then click on Image Size.

3. Lower the Quality of the Picture
If you reduce the quality to about 60% to 80% you can reduce the size considerably. You can do this from File and then choose "Save for Web."A window will come out with the percentage you want to set. Experiment with the settings and see how which setting lets you get a small file size with considerably good quality.
Finally, you can export the picture by clicking on the save button. Click here for a good example of a compressed photograph.