For starters, I have to say that if there's a way to add another picture to your collection, it could be a much nicer composition. That being said, you should really check out the arrangements at the link provided in the article (https://www.potterybarn.com/tips-and-ideas/how-to-... If it must be eight pictures, take a look at the last arrangement example there, called "Centered." As all of the pictures are the same size, I would spread the outer three on each side evenly. You won't have a square, but if each side was a little higher and an equal amount lower than the center two, I believe you'd have a nice arrangement. Another option would be to do two rows of 4, evenly spaced.