ArtRage is a popular painting program for Mac. Its projects (.ptg files) can be exported to several common graphics formats, including JPEG, TIFF, PNG or BMP. It supposedly also supports PSD format for Adobe Photoshop. Hence it's the obvious choice for ptg to jpg conversion.
Unfortunately, it is unlikely that you can find some 3rd party software that would support .ptg files. It looks like to be unique and proprietary file type and thus using ArtRage seems to be the only way to convert ptg to jpg.Open PTG file Open JPG file
A stylish painting package