Is there an easy way to upload a picture to the internet from a mac?

You can upload to Google’s Picasa Web Albums (which I highly recommend) using the Picasa Web Albums Uploader.
http://picasa.google.com/mac_tools.html

Not a Mac user, but a bit of Googling turned up this page on where to select your iPhoto files. Seems like a hassle though, courtesy of iPhoto.

Use Imgur and when you go to the upload dialog navigate to the bottom left of the window that pops up and the “Media” area, shown in this screenshot. If that part isn’t open just hit the little arrow next to the word “media” and it should pop down.

If you use Flickr or Facebook, though, you can upload right from iPhoto with these little buttons on the right side toolbar.

A fast way to share files (also photo’s) is Cloud App. I don’t know if it works in your situation (I don’t use iPhoto) but the app has proven VERY usefull for me. You can always give it a shot:

http://getcloudapp.com/

Where on the Internet are you uploading to? From iPhoto you can upload to Flickr, FaceBook and MobileMe from the Share menu. You can also use plugins for specific services that offer them, or 3rd party apps to grab photos from your iPhoto library.