On what specific websites? Some can be directly accessed via iPhoto. Or you can export the photo(s) you want to post via the export menu or dragging them to the Desktop and then using your browser to upload them from there.
A more descriptive and precise thread title will help cater to the right audience and get you more responses. (see rule #3 under Minor Problems)
To edit your thread title, just click on the
button on the bottom right of your original post and then click the "Go Advanced" button below your message.
Have you also taken a look at MRoogle, since that question may have been asked several times?