I need a cover and posters design for my product

Hi,
I need a cover ( thumbnail image ) and description design for my product to put it on https://themeforest.net/
The product is a control panel for a website. I want an explanation in the design of some properties, such as:
the languages used in programming and the possibilities that possess such as changing color, dark mode, choose the special designs, collapsing the menus, supporting touch screens and displaying statistics, ... etc
Most of the work will be mockups ( PC, tablet, mobile ... ) on which to place the screenshots and simple text over each image. the number of posters maybe 1 or 2 with a large height and the contains will be inside it

Some examples:
https://themeforest.net/item/edumy-premium-moodle-lms-theme/25405424
https://themeforest.net/item/buckle-responsive-multiconcept-joomla-template/21414266
https://themeforest.net/item/discy-social-questions-and-answers-wordpress-theme/19281265
https://codecanyon.net/item/rocketweb-android-web-app-solution-webtoapp/22985174
https://themeforest.net/item/cork-responsive-admin-dashboard-template/25582188
https://themeforest.net/item/digeco-startup-agency-wordpress-theme/26063528

Please, whoever submits should have knowledge of this type of design, as this site does not accept low-quality products
Thanks