When a user uploads a picture use the php file upload function to store and/or resize the picture (for tumbnails).
Add a row in the table Picture. Set PictureId to be primary key and auto_increment - add the url to the picture in the PictureUrl field - UploadedBy should be the same vale us the UserId in the Users table. And a date function is allways good the have, if you want to delete all pictures older then 1 month or someting.
When someone rates a picture add a row in the ImageRating table. RatingId should be primary key and auto_increment - PictureId shoud be the same as PictureId in the Picture table. The rating is up to you what values you want to store. From 1 - 10 or 1, 0, -1.
Use the sum function in your sql database to sum the value of the rating if a picture - if you want to use a 1-10 scale you may also want to count the fields and divide the sum on the count value!