You need to copy the physical files off of your Mac to whatever site you want to host them. Copy & paste cannot do that. Wrong paradigm.
What version of iPhoto are you running? The latest has built-in support for uploading to Facebook and Flickr.
Where specifically do you want to post the pics? Other than your avatar, there’s not much place for pictures “here” on Fluther.
Regardless, Fluther does give a good example on how to physically upload files. Click here and you’ll be on the “Upload a new avatar image” page. Notice that “Choose file” button? Click it and you’ll be prompted to locate a physical file on your Mac for upload.
Where are the physical files in iPhoto? They’re stored on disk in a simple date-based directory structure. I believe (not on a Mac right now) that if you select a pic in iPhoto, hit Command-I you’ll see the Get Info dialog. Somewhere in there is possibly a path to the physical location of the file.—Or—You can drag the pic from iPhoto to your desktop and then point the Choose File dialog to that copy.