Hi,
To do this successfully you would need to add a pix_plugins directory to your theme and add the image in it using this path:
Here is a link to Afterburner theme that uese alternative icons such as SCORM,,,
https://github.com/moodle/moodle/tree/MOODLE_26_STABLE/theme/afterburner/pix_plugins/mod/scorm
There is a short tutorial too that explains the process: Using images in a theme
Hope that helps?
Mary