If you're not concerned about the number posters or their individual sizes, you won't achieve anything by moving them out of the database and into the file system. It will just occupy more disk space, be more error prone, and likely run slower.
If you were interested in reducing the number of posters and perhaps resize them, it may be possible to do so by exporting them (by changing the option to save them outside the database), deleting and resizing, then importing what remains (by changing the option back to save them inside the database). It's been a long time, so I'm not sure that will work. In particular, I'm assuming the image files will be named with some consistent prefix denoting the movie. If that's so, it wouldn't be that difficult to cull unwanted posters from the file system (by viewing large thumbnails), and then resizing the rest.
I might be missing something, but it might be worth a try—
on a copy of your database. And perhaps test the last step before spending a lot of time in the culling.
