Issue » When an image is uploaded in WordPress, different sizes—thumbnail, medium, large—are created based on the settings in Admin » Media. In most cases only 1 or 2 of these image sizes are actually needed. For example you might want to use the large size in the post, the thumbnail as the featured image, and not need the medium size at all. This causes extra resizing work and wastes space in the uploads folder.
Idea » During the upload process let the user choose which sizes are needed—and only create those sizes.