Images are "Faded" Emailed from Outlook 2011 for Mac
I've been trying to figure this out at work and here is what I've learned.

Some PCs are prone to see a faded image when others don't, so be sure to test on a number of PCs (try to find the weakest link).

When a Mac user copies a PNG and paste it into his email signature or the body of an email, this error occurs.

I have to send out a company wide email signature, so here's what we do.

Have a PC user draft the email signature, placing the actual JPEG images. When that email is sent out, a Mac user can copy and paste it and it should work.