Photoshop has a feature called a droplet. It is a drag n drop icon that contains full scripts. You can record in Photoshop as you resize, color convert, and save as file format and it all contained in a sequence. You can set your own custom size determined by your layout of the page.

I use netflix.com for getting 99% of my movie posters. The first time setup was easy once i figured out the droplet. Capture all of the movie posters in one folder, be sure to rename the files you grab into the movie title. I created another folder to store all converted posters (the droplet will even retain where you want new files saves too) Drag all of your posters onto the droplet and Photoshop will convert all of them and put them in your converted files folder.

I've used this a few times for Pronto programming and adding a few posters has become a very quick process