resizing images

I am having a difficult time resizing all my images to all equal the same size. I am using a photo editor ACDSee. I resize images to 1024 x768. After resizing them, I upload into media file, add into photo into Visual editor. The files are all different sizes. What am I doing wrong? Does each file have to be the same image file (i.e. 1MB). Any help would be greatly appreciated.