Drag the swatch from the swatch panel and drop it on the image.
This will only work if there’s no transparency involved.
Select greyscale image with Direct Select arrow (the white one) and choose Pantone Red from fill color dropdown in Control Panel (CS5 or later) or from Swatches panel ( I assume that you have Pantone color picked up to your swatches list beforehand). Make also sure that the black used in your greyscale image is really black, not dark grey...
EDIT: Bob was faster... Don´t you guys never sleep....=)