Nowadays, there is a very wide range of image formats, some highly popular to the average users, some not very common since they are used especially by professionals.
Before distributing an image over the Internet, one often converts it to PDF, in order to make sure the file can be accessed by the targeted user.
Even though it is a proprietary format, Portable Document Format, or PDF, can still be easily opened by numerous applications.
Also, when converting images to PDFs, one can use several graphic formats as the source, without being restricted to a single one.