Changing Block Image Position

I apologize if this has been discussed already. I can't seem to find more information anywhere.

I am creating new plants in my mod. I have noticed the images for wheat repeat on all four sizes of the block and slightly overlap. I also see that Reeds (sugarcane) use just two repeated images but go diagonally across the block. I am creating a plant extending BlockCrops but would like to put in code to show images the way reeds do. Can someone point me to the declaration of how the images are loaded or show me what this code should look like? (This is minecraft 1.7.10 btw)

Thank you in advance. I hope this reads clearly.