To do this your addon would need to download these images and resize them, using both bandwidth and processing power. Depending on the size of linked images and the amount of images your users link to this can be seriously taxing to your server. This is also the reason why image uploads are often limited in size and resolution.
It is possible to make an addon for this, but the reason it's generally not done is simply due to the above mentioned issues (performance & bandwidth). The cons quite simply outweigh the pro's.