I need to fix the width of the thumbnail at 200px, while the height has to be resized proportionally; i've seen that i can fix the width x height for thumbnails, but need to set only one dimension....how can i do it?
From the image styles settings for the "Scale" action:
Scaling will maintain the aspect-ratio of the original image. If only a single dimension is specified, the other dimension will be calculated.
So if you specify just the width to 200px, the height will be calculated while maintaining the aspect ratio of the original image.