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I am not sure, but I recall someone mentioning doing it through the MySQL interface by using an import-script.
Try placing your avatars in ONE directory on your harddisk. Then make a dir listing on the dir and then you get all the right names. Then you have to add some code before and after each avatar-filename which includes the path to the location where the avatars are residing on your site. This should give you an import-script for MySQL.
You could then upload one-two file and then export it from MySQL, looking at the syntax. This should give you a pointer as to what code you should have before and after each file.
The above, however, is not exact, but only how I suppose it could be done. I am sure there are others who can give you more exact directions.
But remember, if you upload 1800 avatars then it will take a long time to open the avatar list when your users want to choose an avatar.
So the question is not really HOW you import 1800 avatars but IF you really want to and if it isn't in reality an exercise in futility, waste of bandwidth and time - especially for your users.