Are you want to allow your visitor or Guest user to post his articles/contents on your WordPress website/blog, User Submitted Posts is the best option for you. This plugin will allow you to offer you users for Guest blogging.
You users can add his articles and images via accessing your front-end WordPress website. After submitted user’s post will be saved in draft for approval from website Administrator. You have an option to approve/disapprove your user’s articles.
You can install this plugin by follow this article How to Install a WordPress Plugin.
How to setup guest blogging
After installed this WordPress plugin your users can easily submit posts and images from the front-end of your website. Below is the screenshot to setup this plugin.