This problem usually occurs when the source file has an embedded color profile that is not being supported by the browser. All browsers render colors of images in a slightly different way, and if the color profile is not supported the colors may look different from what they do in your own image viewer.
You can test this by opening the source file on your local computer in your browser (right click on the file and select "open with", then choose your browser).
To solve this problem, make sure you export your images to web optimized. For example, Photoshop has the "Save for web" option.
For more information on this subject, we suggest this page.