I think that if one had ever come to light it would have been available on one of the online sites that cover the crimes and trial of William Palmer. The events of the 1850s took place at a time when photography was becoming a mass media available to those who could afford the fees, possibly William Palmer could have afforded that, but it is unlikely he would ave paid for his mistress to be photographed. The use of photographs in newspapers was not common then, artists impressions were the way images were represented in newspapers (broardsheets) and so again, there is little to indicate the news journalists were able to or capable of aquiring a photo of Eliza for their articles and reports on the trial.
It is of course possible that a photo was taken in later life of Eliza, did she move to London after the execution to carry on as a maid servant?