Image download handler app
PEP0008 says use 4 spaces for indentation, it's much clearer where one block starts and another ends that way.

This may be a placeholder but I'll assume your code posted here is real code and say that you should just use if blob_info and remove the pointless pass and else.

if not blob_info:
    pass
else:


I also think you could reformat for better whitespace to make your code more readable. You currently use none and that gives a wall of text effect. This looks nicer:

class ServeHandler(blobstore_handlers.BlobstoreDownloadHandler):

    def get(self, resource):
        resource = str(urllib.unquote(resource))
        blob_info = blobstore.BlobInfo.get(resource)

        if blob_info:
            data = blobstore.fetch_data(blob_info.key(), 0, 50000)
            img0 = images.Image(image_data=data)
            img = images.Image(blob_key=blob_info.key())

            if img0.width &gt;= 800:
                img.resize(width=800)
                img = img.execute_transforms(output_encoding=images.JPEG)
                self.response.headers['Content-Type'] = 'image/jpeg'
                self.response.out.write(img)
            else:
                self.send_blob(blob_info)


You should also add a docstring to your class at least. I can't suggest one given my limited knowledge of your script but they're always helpful for a class.