Your question revolves around what is commonly termed Fair Use and, more specifically, what would appear o be a transformative work... and the answer will depend on which country you live in, as Fair Use laws are not universal.
Absent that specific information, a general guideline would be that the more transformative the work is from the original, the more likely you are to be protected by Fair Use laws in your country.
Using the scenario you postulated, I would personally lean more towards hand painting your interpretation of the movie still via oils on to canvas, as opposed to trying to achieve a similar aesthetic via imaging software.
The skill and effort required to paint by hand points to both originality and a "true" transformation, whereas it might be argued that using image processing software requires less skill and thus be less transformative (the "anyone can do that" statement)