Small size (usually used on web pages in order to not slow down the uploading process too much). Thumbnails are always .jpg-files.
Large
Large size which could be used for publication purposes (NB: this size is never of higher quality than the originally submitted file). Large images are always .jpg-files.
Original
Original format as it is submitted. Original images may consist of different kinds of sizes and file types from one supplier to another or within one supplier.
The above formats are just a few options of all available image formats. Click here for the complete format description of images and how to request them.