Black Bars over the eyes and a person's legal rights?

I am wanting to do a collage using pictures of my past and present friends. What I am wondering is, i am not on good terms with alot of the older friends i had but i wanted to use their pictures and black bar their eyes. these girls are not the nicest people and i am worried that even if i black out their eyes they can still claim to have some legal right (saying i am defaming them and such), is this possible? If i have blacked the eyes, put no names on them can they do anything? i am wanting to use it to apply to art school and i have heard that people get in trouble for not having written permission from people they have photographed, is this really true?