A mapping problem for sure. Poser does have problem with long thin polygons, but object does not need to be round.
Since you are using a transmap, the object can be square, the not needed parts will be masked out.
Your image is square anyway, there no round JPEG's.

I gather you are just taking picture of propellor and applying radial blur to it in a 2D program like Photoshop.

You can apply the images to the "one-sided square" primitive in poser and not have to make object.