It would seem that several presentation formats in the past (such as Harvard Presentation) did in fact use .prs files, which means that prs to jpg conversion is doable and might actually be used for several, completely different formats. It is likely that individual screens from the presentation can be saved as pictures, the real deal will be just in finding modern tool that can still work with older .prs files. Of course finding out the original software on some abandonware websites is also option.
Updated: July 18, 2023