I wouldn't recommend this as a way of storing all the users data. However it may be the most simplest way.
You have a database structure like this, where you can track different types of skills available to the user, for this example I only included one skill, and you can keep track of each and every one of them inside a skills column for the user. For items, you can basically do the same, each time a player gets a specific item, you can add one to the Item_1 column, more then one item in the game? Just add more Item columns. . Like I said, this is not the best method.
It works, but ugly.
As for storing avatars, you can't store them inside a database. However, you can let the user upload a image onto the webserver, and store them in a user image directory, and when they log in, call the image file into a sprite.