Image Preview: You can create a 590x300 JPG for your preview image.
Thumbnail: You can create an 80x80 JPG, PNG or GIF for your thumbnail.
Main files: You can save your main files into a .zip folder, including all instructions.
Preview Image Set: You can provide additional preview images. They must be JPGs no larger than 900x900px, and named in the order that you want them to appear using the following format:
01_preview1.jpg, 02_preview2.jpg, 03_closeup.jpg.
Create a .zip folder containing your additional preview images.
More detail can be found from: https://help.market.envato.com/hc/en-us/articles/203269650-A-step-by-step-guide-to-the-upload-process
I hope it will help!