You can save a large amount of space, especially if you have a large number of images..
Add the following to text.txt (format of the IPTC tags taken from here):
2#110#Credit="My Company"
2#05#Object Name="THE_OBJECT_NAME"
2#55#Date Created="2011-02-03 12:45"
2#80#By-line="BY-LINE?"
2#110#Credit="The CREDIT"
2#115#Source="SOURCE"
2#116#Copyright Notice="THE COPYRIGHT"
2#118#Contact="THE CONTACT"
2#120#Caption="AKA Title"
Strip all existing exif data from the image
mogrify -strip image.jpg
Add the credit to your image
mogrify -profile 8BIMTEXT:text.txt image.jpg