Have you ever used the Save for Web option in Photoshop and found your image looks much lighter on the export than on the canvas? It happened to me when I switched from CS on XP to CS 2 on OS X.
The problem is that your canvas is using one colour profile (sRGB is best for web by the way) and exporting using another. If you’re working on web here’s how you fix it:
Choose File > Save for Web and then click the arrow located to the top right of the canvas. Choose Use Document Color Profile – this will ensure the canvas and the exported image match.