Use an external image hosting site like tinypic, photobucket, imageshack, etc. You'll never need to worry about exceeding your quota again ;-)
You may be able to delete some of your images by going to Settings and then Attachments (near the bottom of the left-hand menu bar). Just tick the check boxes next to the images you want to delete, scroll down and hit the "delete selected" button. Some of your images may not have the option to delete, for those you'd need a
mod to delete