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They will have be resized to be added to the site anyway (since >720 pixels wide) so not a big problem for this time, but how can I give hint except that proportions of cars should look good? 
When you remove borders, you should also resize it to 720 pixels wide if it is bigger than that. And for the height, it depends on what ratio the movie should have. It can either be taken from IMDB (not always reliable) or guessed, to find what gives the best proportions. There are few details on http://forum.imcdb.org/forum_topic-1201-How_you_can_submit_pictures.html
When you remove borders, you should also resize it to 720 pixels wide if it is bigger than that. And for the height, it depends on what ratio the movie should have. It can either be taken from IMDB (not always reliable) or guessed, to find what gives the best proportions. There are few details on http://forum.imcdb.org/forum_topic-1201-How_you_can_submit_pictures.html