Preview featured image using frontend post plugin

If you want to show an image without storing it to the server and retrieving that url first, you will need to generate a local url to put in your image tag. That means using javascript. Actually, it’s quite simple (props):

In the place where you want the image to appear, insert:

<img id="temporary-id" alt="your image" max-width="640px" max-height="400px" />

Then modify the file upload field in the form like this:

<input type="file" onchange="document.getElementById('temporary-id').src = window.URL.createObjectURL(this.files[0])">

Now you would need to get that working with Frontier Post. Since you don’t want to modify the plugin itself, you will need to write your own plugin that modifies the behaviour of the parent plugin. That’s possible, but not straightforward, and involves thorough analysis of the parent plugin.