laptops which are suitable for heavy Image and video processing
The answer to such a question is opinion based and the question itself is very broad.

I have used an HP Envy in the past, with a 4GB Nvidia 950M GPU, which worked well with Linux installed.

In general:


The higher the compute capability of the GPU the better (look here at the list for Nvidia GPUs under GeForce products for notebooks). 
Another option worth considering would be to build compute - you will lose portability, but get a vast improvement of power for the same money. 
Finally, you could buy a cheap laptop and then use the rest of your budget on cloud services, such as Google Cloud, Amazon, etc. This might be the best option to get started (with some free credit they usually offer for new accounts) - also if you think you might only be using the GPU for your thesis, and not training huge models for days at a time after that period. If you know you'll be training large models over long periods of time, it's generally more cost effective to buy the hardware.