editing thumbnail

Hi I have tried and read all i can find. I have been unable to adjust the thumbnail to a slider image. I have been on the featured image slider options. There is a place on the upload image page that has a choice of edit image, then the edit page has a choice for thumbnail settings, a choice of apply changes to thumbnail, then I go to the slider image and crop it, it wont let me save the change or apply it to the thumbnail image. -- please help, the thumbnail image is of the center of the slider image and visually makes no sense... i just need to adjust the image to be to the left... -- thank you in advance...