Step 1: Open the image you wish to examine.

Step 2: Zoom in until you can see clearly the area for which you wish to determine the RGB values.

Step 3: Use the Eyedropper (Sample) tool to change your selected color to the color you wish to examine.

Step 4: Choose "Edit Colors" from the "Colors" Menu at the top of the screen, then press the "Define Custom Colors" button. The color you selected should be displayed.

See the boxes marked "Red", "Green" and "Blue"? Those are your RGB values! Feel free to use my [url=http://mortal_elf.tripod.com/automagic.htm]new tool here[/url] to determine the appropriate DMC floss, or manually search here.
-Thanks.
/Mortal_Elf/