Don't get frustrated! Once you get used to it, you won't want to switch back!!!
First off, which Operating system are you using? If it is a brnad new iBook, it will have OS 10.3.
When you click on "Upload Photos", click on Choose File. It will automatically bring up your Finder window. On the left hand side at the top you should see 2 icons. One will say network the other will say the name of your hard drive. If you click on your hard drive it will give you a list of all the folders on your drive. You need to navigate your way to the proper folder.
If you know the name of the picture but not where you saved it, you can do a Find for it. Click on the Finder icon in your dock at the bottom of your screen. it is the one that looks like a face. When you do that, it should say Finder in the top left corner of your screen. Click on File in the top toolbar and go down to find.
Now that I have gone through that, I can say that the most logical place to save pictures is in your Pictures folder inside your User folder. A quick way to get there is to double click your hard drive. Again, that brings up your finder window. On the left hand side, you should see some icons. Click the one that looks like a house. That is your User Home folder. Inside that you will see your personal documents folder along with your pictures, music, movies folders.
The Mac OS is great. I switched to Mac about 5 years ago. Like I said above, once you get to know how it works, you will find it much easier to use than windows. The Applications Apple gives for free like iMovie, iTunes, iPhoto make using a computer amazing! If you are going to save lots of pictures, I highly recommend using iPhoto. You will never have a hard time finding a picture again!
Send me a private message if you have any problems. I would be more than happy to help you or any other Mac users out!