Corel Photo-Paint 9 is a great tool for making a mosaic from digital images of the human Retina. The images are aligned by putting a semi transparent image over another image and matching the blood vessels. The image can be rotated and stretched to compensate for optical aberrations when photographing a concave surface. The most useful feature is the transparency tool that works like a paint brush for removing the sharp edges of the image. The borders are simply brushed away, changing the attribute of the transparency brush can put the erased image back on. This feature is possible since the change is done to the mask of the image, not to the image itself.
This is a mosaic of Choroideremia disease in the human Eye.