You may be referring to floaters. To wit, pieces of cellular matter that have sloughed off the inside of the eyeball and pass between the retina and the optic nerve. I have them; lasers can clear them if you so wish.
On the retina.
why isit difficult to see the details of images that are formed at the edge of the retina
Retina is a part of eye where images of objects are formed.
Retina
brain
The retina is where the image is formed.
behind the retina
The cornea and lens of the eye form a real, inverted image on the retina.
The image should form on the retina. More specifically in the fovea centralis of the macula of the retina.
It can't. Lololololz.
Myopia
The inverted or upside-down image is formed on the retina.