I simply want to upload an image (JPG) using a form, then send that image to Rackspace \'Cloud Files\' or Amazon \'S3\'.
How about ignoring python all together and just uploading directly to s3?
You can configure your s3 bucket to disallow uploading any files larger than $X bytes.
Here's a simple example to illustrate uploading directly to s3 (and ignoring your image width/height conditions)
http://sente.cc/upload_to_s3.html
code:
refresh the page after you've submitted to see your new image
http://s3.amazonaws.com/dev.sente/image.jpg