On a product, service, labor, allowance, or package, if you don't add an image, you will see the default D-Tools Cloud image in the proposal. While this looks better than an empty white square, you may want to customize this to display a better image that more reflects the item or your brand.
To add a image to an item, you can do so in the catalog or in the quote.
To add an image from the catalog, select the item you want to change. Under the title, there will be a section for pictures.
From a Quote
When in the quote, you can click an item so the sidebar shows up on the right side of the screen. Here go to the actions dropdown and select "View catalog item...". Here you can repeat the process of dragging in the product like in the gif above.
After you have added the image, you will see the image for that item in the appropriate places, like the catalog, when searching for products, ...
...and in the proposal for your client.