Sharpness Lost In Mac Preview while saving

I am using 10.6.5. I usually like post processing my photos in Mac Preview. I open a photo in Preview, and go to Tools-->Adjust Color--> increase sharpness to make it look crisp and save it, I see the image looses the modified sharpness. However if I change other factors like Saturation, Sepia etc they are saved properly.

So I end up, taking a snapshot using the Grab tool, but that also reduces the picture size, quality.

I am open to use any good alternative free/cheap image processing tool. Any suggestions are welcome.

Thank you,
Himalay

Free GIMP - for Mac OS X

Relatively inexpensive - although it seems to have risen in price Pixelmator

A little cheaper than Pixelmator Acorn, a great Mac image editor, built for humans.