I hope you are well today and thank you for your question.
Currently, there isn’t any option to add an image in the review but we will consider this to be implemented in the future version of the theme.
To achieve this now, you have to develop custom code in the child theme of Gillion theme.
Developing custom code for custom functionality is beyond the scope of support that we provide here. Please see https://themeforest.net/item/gillion-multiconcept-blogmagazine-theme/19470306/support
If you are not a developer then you can consider hiring a developer to develop it for you. You can hire a developer from any freelance site. Shufflehound recommends the developer https://www.upwork.com/freelancers/~011652ffec8865c6d5