You have to do this because most of the layouts look good with a small number of pictures uploaded, but how will it look when you will have an odd number of pictures to show? There needs to be a balance between the number of pictures on the page and the negative space you leave between or around them. If you have many pictures, consider using pagination or categories to separate them and do it for the sake of the user. Donandt forget that they never saw you pictures before and if you are really interested in them, it takes a lot of effort for the human eye to analyze them. Donandt make it even more difficult for the visitors.