Trouble uploading pics to walgreens and other places from iPhoto.

First of all, Mac is new to me so please go easy on me and explain things to me in the simplest form:)

Okay, after many internet searches, I learned how to import old photos from our previous computer into iphoto. I really like the streamlines easy user-friendliness of iphoto. However, now that I am wanting to print some of these old photos I cannot seem to get access to the pics when the sites ask me where to upload photos from. I go to my finder, click pictures, click iphoto and just wait and wait and wait. I have waited for hours and still nothing appears in the window! I have alot of photos and wonder if that is whats preventing them from showing up. I transferred over 29,000 photos...

Is there a better way to access my pics when wanting to upload them for print? Am I even doing any of this right-should I have even put all of my pics into iphoto or should I store photos by year on my actual computer and just use iphoto for a year at a time and then transfer the photos to my actual computer?

I would love any feedback or help!

kourton