Supposing that we are talking about this library:
http://www.vividsolutions.com/jts/JTSHome.htm
it serves the purpose of manipulating geometric shapes in Java.
Examples: determine if two shapes intersect, compute the union of several shapes, perform operations like buffering, simplification and so on.
This library is mostly used in Geographical Information Systems where information about, say a county, is recorded in a shape like a Polygon. By using this library one could infer whether and how much of a county falls into a special area (this special area needs also be a Shape) by performing an intersection.
Since you are talking about pixels I would say that this library is not of much use to you. Raster data (that is images like pictures or satellite imagery) cannot be processed by JTS and require special raster processing software.
Depending on the kind of processing you need to apply to your images I might be able to suggest a software, but you need to clarify.