It is impossible to give an accurate appraisal without seeing the item in person. This is partly because you might post an image that might actually be of a print. Also, there are many factors to take into consideration such as condition, provenance, and general desirability. Sites that purport to give accurate appraisals via email can in fact only give you an indication of a typical auction price for that specific artist.
I am often asked about providing appraisals, and even were it to be possible via the internet, I would point out that picture research is a time-consuming and painstaking process, and you are unlikely to find someone with the right expertise who is prepared to provide their services free of charge.