The images on my webpage appear fine but when they're converted to the PDF they are all faded and dull;
They're about 33% brighter than they should be. I made a solid black (#000000) image just to test it, the colour that came out in the PDF was #4E4E4E.
This happens regardless of whether the JPEG has an RGB or CMYK colour space.

Is there some way to prevent Prince from treating the images to make them brighter?

Prince embeds JPEG images exactly as is, so there should be no difference. Can you attach a sample image and PDF to this post, or email it to me? (mikeday@yeslogic.com)

Ok, I've figured out that the images aren't being brighten after all, they're just semi-transparent on a white background. What I'm not sure of is why they are semi transparent.
I found that if I changed the image's position to 'relative' from 'absolute' that they are no longer semi-transparent, but why should this matter? They appear fine when viewing the base html page, the transparency only changes when exported to PDF.

I haven't been able to replicate the effect in an example PDF, the images display properly. It's only happening in the product catalogue I am designing, which is too large for me to upload.
I will keep trying to isolate what's causing the issue and hopefully can produce an example which exhibits the odd behavior.

Here's the CSS for the images:

.product .photo{
	height: 75%;
	left: 12.5%;
	top: 4mm;
	position: absolute;
}

When I change them to position: relative; they appear full opacity, but the sizing and positioning is not how it should be.

There should not be any transparency or other images changes dependent upon their position. Could there be a semi-transparent block drawn over the top of the images perhaps?