so my project is about handling images.
we have back end codes in c.
now we need a GUI..
in what language should we create our GUI so that we will be able to connect our c code with the GUI?
swing , vb.net??
i have no idea

you can do it in C, C++ or vb.net, use which ever language you already know and is able to call C language functions (which is most languages I know of). A more relevant question might be what gui libraries do you want to use. In c++ you could use wxWidgets, in C or C++ you could use OpenGL. It all depends on what you want to do with the gui.