I don’t think single photo observations are in and of themselves a problem. Some organisms are easily identifiable with a single image of varying quality. If there is a particular taxa you are attempting to ID but one image isn’t enough you can comment that and request more images or suggest which parts of the organism need to be photographed for identification.
Additional functionality I would like to see is requiring certain images for specific taxa (ex: tree identifications need bark, leaf (if possible), overall image of the tree, fruit, flower) but that is just incredibly unrealistic. So the best option is to just inform people when you come across the observations where one image isn’t enough.