Every item that you buy should come with both a license to use said item and a read me page . some license's have restriction but if you create an image and post it of create a comic and wish to sell that comic, you can do so because you have a license that allows you to use that item in a commercial venture.
Some licenses will have restrictions especially where Animation is involved, there may be a secondary license you may have to purchase so read both the license agreement and the read-me file that should mention the use for animations.
the read-me file should contain the email address of the vendor so that if you have questions you can contact them.
Freebies may have different sets of rules.. some allow you to use the item in anyway you wish, while some freebies come with a read-me that may say, not for use in a commercial venture without permission. Should you wish to use the item to profit from via a set of images, a comic, or an animation, you will need the creators OK before doing so. Most say yes, but always ask first.
As to using one of the actual images that was used to promote the item you bought.. that may violate copyright as the Vendor may have used other items or backdrops he or she created with the licensed props they purchased to help promote their own item..
Who knows what lurks in the hearts of perverted artist....
The Shado Knows