Using the export command in iPhoto allows you to select the size you want the photo to be. Export to your desktop then upload to flickr or where ever you want.
You can get a CS2 Pshop cheap now (try ebay) or a new Elements for a good price that does batching, automating seamlessly and handles converting to jpeg wth a decent preview so you minimise pixel loss when compressing. The free ones don't do this. I frequently get over compressed jpeg from clients that lose pixel information that makes them rotten for anything other than small sized online work. JPEG is a lossy compression format..
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