The only program that is capable of working with .ptg projects is ArtRage. It should support import of JPEG images just fine, so technically capable of JPG to PTG conversion. However it is not exactly a conversion, more of a working process.
Convert JPEG bitmap image format to ArtRage project.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert jpg to ptg on Windows
A stylish painting package
Convert from jpg file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.