The problem is that File Manager isn't associated with search which only relates to entries, so when you add an image to an entry, only the URL is stored with the entry, therefore only the URL is included in the searching.
The simplest solution would be to use a matrix field for your file uploads and include a column for the IPTC keywords to be added manually.
For example, in the below grab I use matrix to store information relevant to an image (alt text, title, etc) and ensure that "Is field searchable?" and "Is col searchable?" are ticked...
The alternative is a big task! Search is performed by looking at the field_id_x columns in the channel_data table. So if you simply place the IPTC reference in there, it will conflict with the fieldtype or entry data (e.g. a wysiwyg field).
So you'd need an extension that fired when an entry was saved and used:
entry_submission_absolute_end and publish_form_entry_data to capture the save. You'd then need to parse this data for the relevant image paths, read the image data extracting the IPTC information.
That's the easy bit.
Then you'd need a custom lookup table in the database to store this information and reference the entry ID and URL.
But all this is still not going to associate with the search process in EE - and that's a problem as there's only the ability to change the search results (channel_entries_query_result hook), not change the SQL used to search on. So you then have to rebuild the exp:channel:entries tag pair to customise the SQL, run the search and then tap into the channel class and manually trigger the entries method to parse the parameters and output.
If you were to do your own sql query to do the search output, you'd still need to have saved the IPTC data and need to parse the data on output and process any parameters associated with the normal channel:entries tag pair. Although your custom SQL could output a list of entry_id's that you could then pass to the channel:entries tag to let it parse the output for you.
It's no small effort, so paying a junior for data entry would look far more appealing :)