Since you don't know where the cells are, cropping the images could mean deleting the areas with the cancer cells. So it is your decision to crop, since I can not see the images, nor am I specialized in endometrial cancer...
As to how to detect them: first you need to know how the cancer cells are distinct from normal cells (color? morphology?).
On a final note, there are many books on segmenting images, including the use of neural networks.
