ON1 Apps do not not support Grayscale or CMYK images.
These type of images can often be created scanning old black and white photos, or if the file was saved in Photoshop in the 'Grayscale' or 'CMYK' color mode.
You will need to use another app like Photoshop to convert your images to RGB in order to be able to use and work these images.
ON1 does not have the ability to change Image Modes to convert your image from Grayscale or CMYK to RGB.
In Photoshop you can open up your Grayscale or CMYK image and then goto the Image Menu>Image Mode and change the Image Mode to RGB.
Once you've done this you can save the change and your files will be recognized by ON1.
If you do not own Photoshop or another photo editor that has the ability to change Image Modes and convert Grayscale or CMYK images to RGB, there is a Free Open Source alternative app called GIMP