Based on the facts above, it seems that you are creating a work that includes your original contribution, namely your images and ideas. However, your contribution is not standing on its own. Generally, celebritities have a "right of publicity" in their own likenesses and identities. Your contribution apprears to be a use the underlying right of publicity. It appears that no permission for that use has been obtained. Your contribution of an image does not make up for the lack of permission.
I think you recognized this when you said, ". . . I don't own license rights to Jason Vorhees . . ." More business analysis aand legal analysis would be needed. However, many people would shy away from selling such images.
This is NOT legal advice. There is no attorney-client relationship. Consultation with appropriate counsel regarding all relevant facts would be required.
To give you some background information on licensing celebrity images see the links below:
Celebrity Images Rights in Law
The Use of Celebrity Images in Works of Art
California Publicity and Privacy Rights Act
Any time you use IP that belongs to any one else, you're creating a "derivative work," which only the original creator has a right to create, so there's really no such thing as "fan art." Generally the test for infringement is "substantial similarilty," which means if your art looks substantially similar to their art, it may be too similar, and therefore infringing.
If you create your own art, drawn yourself and with your own ideas which is NOT based on anyone else's art, you won't have that worry, and can put it on shirts, posters, etc.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
To clarify, Jason Vorhees is the Jason character from Friday the 13th. He is obviously not real, and therefore the issue here is not right of publicity. Rather, copyright is the primary concern, and I'm sure trademark to some extent as well.
Your fan art is a derivative work of the movies, and derivative works are an infringement. Creating a derivative work for your own personal enjoyment and use could be considered a fair use exception to infringement. Commercial use, such as by selling the derivative work, is a strong indicator of infringement. The exception here may be if your fan art is a parody of the movies, but I'm assuming that is not the case since you didn't mention anything, and "fan art" sounds to me like a tribute of some sort rather than a spoof on the original.
Always consult your own attorney who can provide you with legal advice.
Disclaimer: In no way is this information considered a legal advice, but is only a statement of the law. This information does not form a client-attorney relationship.