Trigger whenever a user uploads an image
If you're using a field field in Drupal 7, you could accomplish this with the use of stream wrappers, core-supplied file API hooks, and a file field.

See the file examples in the Examples for Developers module. Also read through the 'Stream Wrappers and You' post from Advomatic for some background.

For more advanced triggered actions, you could also consider using the Rules module to react to a file being uploaded to the site via a new node or other entity type.