You can choose to compress picture files directly or to create a compressed copy of them.  If you choose to compress files directly be aware that once Quicktime compresses a picture some information is irretrievably lost.  How much information is lost depends on the image quality settings chosen.
If you choose to create a compressed copy of the picture files, you can specify the suffix added to the end of the compressed copies.  For example, using the suffix “.jpeg”, a picture named “Violet” will be changed to “Violet.jpeg”.  If there is already a file named “Violet.jpeg”, the compressed copy will be called “Violet.jpeg 1”.