Hi there,
Great to see that you would like to upgrade the speed of your websites. This is also one of the SEO factors which Google uses to decide on your place in the Search Engine Results Pages (Short for SERP).
I would love to help you with this.
First of all I would optimize the image for usage in your webbrowser next to that I will optimize some parts of the code of Opencart.
Just one thing that I would like to note here. The speed of the webpage will not be faster if you put the images on a subdomain. Especially not if this is all hosting on the same server.
You know, any subdomain is linked with a folder on (the root of) the server. The webbrowser requests the webpage and all the server will do, is forward the images and HTML / CSS to the webbrowser from the same destination. The page won't be optimized for speed by doing this.
Yet, by optimizing the code and pictures, the webpage can easily speed up. Next to that I have some other trips and tricks you might be interested in. For example; changing hosting can change a lot!
If you want to know more, please drop me a pm. I would be able to finish this up in under 4 hours for you.
My bid is way below my average, but as I am new here on freelancer (as a freelancer and not as an employer!), I have lowered my bid.
Thank you for reading through and would love to see your message.
Thanks,
Stan