Hi Stefano, at this time it is not possible. If you upload to facebook, you can simply use the when posting a photo here and link to the image. No need to upload to the forum as an attachment. Attached images are placed in the file system and can not be automatically re-sized.
To make it easier to re-size images, I have added this online photo editor to the forum. I hope it will help. It seems very speedy. I have posted a step by step demo video on the OrchidTalk Help Video Page for anyone who needs information on how to use the photo editor.
Cheers,
BD