I want to make a diagonal/ triangle shape image from bitmap source without using any polygon control, how to do this in windows phone.
Hm what do you mean without using any polygon controls?
You can create a WriteableBitmap, and draw on it with any tools you prefer. If you want to avoid all additional tools you can call set/get pixel methods and for loops.
When you are happy with your WriteableBitmap you can create a BitmapImageSource and then call .AsBitmap() on your WriteableBitmap. With that complete you can then use the whole imaging SDK chain.
Edit: You can find the .AsBitmap method in Nokia.InteropServices.WindowsRuntime namespace.