headfonia, yes.
ronenbekerman, if you post a thumb image to the "Thumbnail Image URL", then you have the thumb posts covered. For the 528px (content width) images on the frontpage of the blog, there the feature size images will be used. I could add a additional image url field for those too though. Automatic is best, but if you have to upload the images to an external server, how automatic can it be. You can use an uploaded image for the thumb posts though. Add a local uploaded url to the "Thumbnail Image URL" (it then will use timthumb) and an external for the feature and content width images.