Form validation early on (preprocessing images)

Given a form where users should upload some images, which'll processed, validated on form submission. Unfortunately, this results in significant overhead (time) on the 'thank you' page.

I'd like to minimize this overhead, and I'd like to process these images as soon as the user selected them.

Is this a good or bad pattern?

First and foremost, the process time could be eliminated entirely, but on the other hand, I use their data plan without their consent, even when they abandon the form.

Which one would you prefer?