What it means: Photo or other image that is grayscale, but saved as a color image. This results in an image building gray from all 4 colors instead of just using black.
Problems caused: This can cause the image to have slight color tints. Additionally, the image file itself is larger than it needs to be, as it is using 4 color channels instead of just 1 color channel. Using grayscale for all black & white images will help keep your PDF file sizes down, making them faster to upload.
How to fix: Convert the photo or image to grayscale unless special color effect (such as sepia) is being applied.