How to add photos from thousands of folders to iPhoto?

I have an old hard drive with thousands of photos separated in thousands of photos (a folder for every day, don't ask why I don't know). I just want iPhoto to grab all images out of every folder and add them to iPhoto library. What is the easiest way to do this? I can't seem to just add the main folder.

Additionally, some of the folders have other files (pdfs, docx) in them.

How to I save myself from manually opening and adding tens of thousands of photos from thousands of different folders?

Thank you in advance.