If the image is not in the public domain--which is very unlikely for a movie image since, as a rough rule of thumb, something needs to be 75 years or older  to be public domain--or you do not have permission from party which owns the rights to the image (e.g. the studio), you cannot post the image on your website. Movies, and stills or images from them, are copyrighted; certain images may be trademarked as well.  If you put it on your website without permission or without it being public domain, then even if you properely credit the image, you have  violated intellectual property and could be sued.It may be that as a practical matter, no one would sue you--but be aware that they may have the right to do so if you are using their intellectual property without permission.(There are some exceptions to copyright protection--e.g. satiric uses; de minimis uses for noncommercial purposes--but a good rule of thumb is that if you have a website that is commercial in any way, do not use other's intellectual property without permission.)