Guillotex on "Get Post Feature Image on my own homepage"

I built a homepage for my WordPress site from scratch, copying the style of the theme Reflex (which I actually bought). I'm doing this because this way the site is faster, and use a lot less bandwidth.

Original site and my build

But, I have trouble getting the featured images, I did something very 'strange' just to work it out, this is the code

<img src="'.$thumb_img.'-200x200.jpg">

I add "-200x200" at the end of the link which I got from the database. I understand this is not the best way to do this, so the question is:

How should I solve this?

Thanks in advance!