The characters you are seeing are what happens when you do a dump of non text data from the database (in this case avatar pictures). There are a couple of solutions.
1) Toggle the settings in your ACP so that all images are stored on disk instead of in the database. This will remove the need for the avatar table altogether
2) I have never used mysqldumper, but the program it is based on (mysqldump) allows you to set --hexblob so that binary data is hex encoded (removing the weird characters, and most importantly the quotes). I assume mysqldumper will do the same.