You own the rights to the photo. You might not have the right to use the photo without restriction if the photo is deemed to be a derivative work of Reaper's copyrighted sculpture.
It is possible that a painted miniature is a sufficiently transformative use that photos are fair use in any circumstance. Further, since the sculpture is sold with the express purpose of being used by purchasers to create original artwork, it is possible that Reaper has constructively waived a small portion of their control over that sort of copyright.
Only a judge and jury can tell you for sure and only a lawyer specializing in copyright cases can give you worthwhile advice. I am not such a lawyer, or indeed any sort of lawyer, so the above is just my lay supposition.
If you make an arrangement with Reaper in advance, none of that really matters, though, and it's likely that such an agreement would be both easier and cheaper than even winning a case would be.