The American College of Optometrists has a page on this alleged phenomenon, called "optograms". They claim that with chemical fixatives and immediate extraction of the retina, one can discern a rough image representing the last image exposed on the retina. This is vaguely plausible, since the formation of an image does induce chemical changes in the retina.
In the mid-17th Century, a Jesuit friar named Christopher Schiener did some experiments on frogs. [Ref]
The College go so far as to say:
The one and only case of a 'Human Optogram' is therefore that of Erhard Gustav Reif in November 1880....[H]e was executed by guillotine in the prison yard in the small German town of Bruchsal. His left eye was extracted within ten minutes of the sentence being carried out. Reif's optogram, some 4mm in height, does not survive, merely a simple sketch drawing taken from it.... It has a superficial resemblance to a guillotine blade although the victim's eyes were bandaged seconds before the blade fell. Possibly they are the steps he had to ascend shortly beforehand.
In 1975 a group of German scientists in Heidelberg reproduced the original rabbit research. It seems that they were able to produce rough images, but concluded that the technique had no forensic or scientific value.