Its called a Cheese Grater.
You need to import them into iPhoto to get them to be recognised by that app.
Move your iPhoto Library to your other disk, the library is in your Pictures folder by default.
Hold the option/ alt key down whilst opening iPhoto and you should get a dialogue asking you to Choose Library
select the library you moved to the disk, iPhoto should populate with your images. Once you are happy your images have been imported correctly you can delete the original iPhoto Library which will free up space on your start up disk.
Now you can import your jpegs from the folder you moved earlier, once that has been done and all your images are now in the iPhoto library you can delete the original folder on your startup disk and the folder you moved to your other disk.