Image Masks not downsampled?

[AnotherMe]

Can anyone explain what this message means?

Thanks

[Test Screen Name]

I'm assuming you know what downsampling is, that you asked for it, and you know the consequences of not doing it. So, what is an image mask? In a PDF it is a kind of 1-bit image that has no colour information, but can be used in any colour. Often used for special effects or fonts, so some software does not touch them.