Categories image by default is shown only when you view the category page. You can configure your template (M2 or Kinabalu) to have the images to show in index page:

1. Login to the back-end
2. Go to Mosets Tree > Templates > Kinabalu (or M2 if that is the template you are using)
3. On the right hand side of the page, you'll see a list of parameters. Set "Show Categories' images in Index" to Yes. This will display your category images in Index page.
4. Click the Save icon on top right of the page to save the changes.