(From 11 May 2016 the Supplemental Classification is replaced by Image Type Tags:
see http://www.geograph.org.uk/article/Image-Type-Tags-update ).
If an image doesn't quite fulfill the Geograph criteria above, but is still a good image, we'll accept it as "Supplemental image" - no Geograph points are awarded, but the image will still appear on the selected grid square. A square that just contains Supplemental images is still open to be claimed as a Geograph though!
Supplemental images are not inferior to Geographs, just different. For the interpretation of a square they are just as valuable as they add extra information (in the case of close-ups) or wider views (in the case of aerial shots or images taken from another square).