There are a few things you need to do before jumping into Envatitor. First off, create your images that you want them to link to your other items. Once you have your images ready, upload them to an image hosting website or Dropbox (I use https://postimage.org) and get direct links to them.
Copy the following peace of code, paste it in your item description and replace the placeholders with your links.
<a href="YOUR_ITEM_LINK"><img src="YOUR_IMAGE_DIRECT_LINK" /></a>
Hope that clears things up a little bit!