From there we look at condition. A rare title may be worth little if in poor condition. Condition includes not only the quality of the picture itself, but the mat and frame also. Since the mat contains the signature and sometimes the title, it is an integral part of the total package. A frame can be replaced, but many collectors are purists and they feel an original frame in good condition adds to value.
Size also plays a part in value.
Larger does not always mean more value, but many times does. A titled picture is generally worth more than an untitled picture. Condition is so important. From major problems such as tears in the picture or mat, to more minor problems such as spots or dirt. Color and brightness are looked at.