The quickest way would be to "drag" the photo from iPhoto onto the Desktop. This will actually make a copy of the picture, rather than removing it from iPhoto. You should then be able to find the picture to upload from the Desktop rather than trying to find it in iPhoto, which is a little harder (it's actually filed in Home>Pictures>iPhoto Library). Just trash the photo on the desktop afterwards.
If the photo is quite large in size and you need a smaller version for the web, you can select the photo in iPhoto and go to File>Export and save a copy as a smaller version.
Hope that helps,
Applemacster