It sounded like your image might have inverted colors, so I jumped into your campaign to take a look. I found that your image was saved with CMYK color values, which are for print, instead of RGB colors values, which are for email. You can change the color values to RGB using a program like Photoshop or a website such as picmonkey.com and then upload the new image to your campaign. That will do the trick!
I'm editing your topic's subject line here because I know other folks have this question, too. :)