I understand. But what you did, or tied to do, was COPY the pictures from iPhoto to your Flash Drive.
So, two things to do:
1. Open iPhoto and make sure they are still there. They certainly should be. Whatever you did doesn't MOVE them.
2. Open your Flash Drive (Mount it, and double click on icon to see contents). If your copying worked, they will be there too. Confirm they are there.
From the above, you should find original photos in iPhoto and, hopefully, ALSO on your Flash Drive.
If that's the case, you have 2 sets — one in iPhoto, and another on Flash Drive. You can delete from Trash.
However, fear not. It is quite easy to return them from Trash to wherever you want. No point in doing this if they are still in iPhoto - just creates duplicates and/or complicates things.
Post back what you find, please.
Ian
PS It is late night where I am. Could be tomorrow am before I get your post — unless you're very quick