How to create Thumbnail which contains high Resolution photos ? or Any extra tips and tricks ?

I just join Envato Market .
I have already created 3-4 nice Template but Im facing problem in publishing Which is

" So , Envato wants my template design in Thumbnail Format for showing my template design on Search items

But , whenever i create a thumbnail its Photo resolution and quality decreases .
and it looks poor in Quality "

and It will be great If I could get Someone’s (Aurthors) Contact no or contact details for tips and instructions and Cause I have 4-5 questions related
Envato !

Thanks you !

Make sure to read this

Envato wants my template design in Thumbnail Format

That is not true. Thumbnail is a small 80x80 image, nobody expect you to have template design fit into that in any way. Just open any author’s portfolio and you will see how thumbnail should work.

I assume you actually meant preview image. That’s 590x300px image, but it doesn’t have to container your template design at all. I personally make mine from my item’s screenshots, but that’s my preference. It is up to your creativity how you handle it. Again, just take a look around marketplace how preview images can look.

If you have any further question then just post them here.