halluzineyt,
I don’t think this is going to be possible. I didn’t quite understand what you meant when you were requesting this, but it appears to me that you are just putting a url/link to an image in the custom field, correct? If so, those could live anywhere, even on another server, and there is no way that I can think of for my plugin to determine the actual physical path of the image that needs to be optimized. Just to be sure, where do you actually store the images that you link to in the custom field?
tresero,
again, please post a separate feature request. I think I have most of the pieces in place to implement it rather easily, but I won’t spend my time doing so until you prove it is important enough by following instructions.