Adding still frames to a movie page

Having entered recently a movie title, from an obscure Mexican producer, I wished to add three still frames I rook from the movie. I downloaded the full movie from a social media, when the producers had it on line for free, and currently they only have the trailer on line. 

Today, when I added the first of the three pics to the movie page, it uploaded well, then I chose «still frame» for description, I did not add the copyright info (that is not mandatory, and which I have doubts about), entered the name of the actress in the pic, and befor I could submit it, the screen jumped and the image disappeared.

Three attempts with the same result is not an occasional IMDb malfunction, so... what am I doing wrong?

By the way, the image is 335 kb JPG.

Thanks.