thumbnail and preview image

How to make theme thumbnail and preview image without losing quality.
Please answer my question!

Are you talking about thumbnail and preview images for an item submitted for sell on ThemeForest?

Yes,you are right

Can you please explain what do you mean by “image without losing quality”? Thumbnail and preview are two different images which strictly set dimensions, why should there be any loss of quality? I mean loss of quality compared to what?

So,I do’nt know,by which program I can make Thumbnail.

Any graphic editor like Photoshop, Gimp, etc. Anything you can use.

Make sure to read this article:

Thank you very much,it’s helpful.