Malcom - The usual reason is that photos on the web should be posted in the sRGB color space. You're probably working in Adobe RGB, so you need to convert your image before posting.
This is because your browser is not color aware (well, probably not - Safari is) and will show colors differently.
Malcolm,
In PS Go to the "Edit" drop down menu, select "convert to profile".
The convert to profile box will pop up. Select SRGB in the "destination space" area.
Click save.
And then save for web.
I hope this helps.