If you look closely at the OpenFileDialog class, you will see it has a Multiselect property
] - set that to
and it will allow the user to select more than one file at a time. You can then use the Filenames property to open each image in a loop.
Do be aware however, that you should Dispose of Bitmap objects when you are finished with, don't just let them go out of scope or it will cause problems later.
BTW: You do not have to call ToString on string objects such as OpenFileDialog.FileName.