I suppose it’s the same as saying good, better, best or the reverse alternative bad, worse, worst. It all depends where you start from. Pretty, Beautiful, Sublime at least allows you to not be too harsh with your opinion of a picture, it’s purely a question of semantics.
PRETTY allows you to damn the image with faint praise. Whilst you’re not saying you consider it awful or terrible, you are implying it really has nothing to commend it apart from a picture postcard or chocolate box image, and it does nothing to evoke any feeling within you. Now, if that is what the author has in mind to portray you’ve pleased him/her immediately without you betraying your true feelings. It’s difficult to categorise what pictures fall into this category but they usually comprise travel/holiday images, record pictures and family and pet shots.
BEAUTIFUL can mean that the image has caught your eye and makes you want to look closer to see what there is within it that makes you so interested. The tones the different composition, the photographer’s management of the subject, the viewpoint and the overall way the photograph is presented. It is a picture that has impact and is quite memorable.
SUBLIME means all that is included in beautiful to such an extent that you have an overwhelming feeling that you wish you had taken it yourself!