Fixes #27 resized images not uploaded

Fixes #27 .
Changed the filter used by the plugin to do the upload, from wp_generate_attachment_metadata to wp_update_attachment_metadata. 

Also, modified the check for the original file to exist to allow the execution to continue uploading other image sizes.

I just noticed that when uploading additional image sizes it may be good to check for those files to exist. As it is, it worked without issues though.