Sign up or Log in to your Canva account.
Click on "Create a design" (top right corner) then click "Custom size".
Add the size of the image you'd like to generate (e.g. 2000x1000px)
Upload/select the image file you'd like to use in your mockup
Click the Image [6] > Effects [7] > Smartmockups [8] (select "See all").
Once you've chosen/adjusted the one you want to use click download
You're all done. Enjoy!
Tip:
Create as many mockups as you wish but don't overuse it on your website because your business will feel "too stock" and customers won't like this.
Here is a few more similar websites you can use for more FREE/Paid OPTIONS (frames, mugs, shirts, billboards, computers, walls, logos...)