You can’t just put a File directly as the parameter for background-image. You started using FileReader but didn’t do anything with the result.

Here’s what I would do below, I converted your images array into a list of objects that have a file property and a preview property, just to keep the file and preview linked together.

Vue.config.productionTip = false;
Vue.config.devtools = false;

new Vue({
  el: "#app",
  data: () => {
    return {
      title: 'Select your images first and then, submit them.',
      images: []
    }
  },
  methods: {
    selectFiles(event) {
      var selectedFiles = event.target.files;

      for (var i = 0; i < selectedFiles.length; i++) {

        let img = {
          file: selectedFiles[i],
          preview: null
        };
        let reader = new FileReader();
        reader.addEventListener('load', () => {

          img.preview = reader.result;
          this.images.push(img);
        });

        reader.readAsDataURL(selectedFiles[i]);
      }
    },
    uploadImages() {

      const data = new FormData();
      data.append('photos', this.images.map(image => image.file));

      axios.post('url')
        .then(res => {
          console.log(res);
        });
    }
  }
});

.image-result {
  width: 100px;
  height: 100px;
  background-size: contain;
  background-repeat: no-repeat;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app" class="content">

  <h1>{{ title }}</h1>
  <div class="btn-container">
    <label class="button" for="images">Select Images</label>
    <input type="file" id="images" name="images[]" multiple="multiple" @change="selectFiles" />

  </div>
  <br><br>
  <div class="block">
    <h3>Selected images</h3>
    <div id="target-photos">

    </div>

    <div v-for="image in images" class="image-result" v-bind:style="{ backgroundImage: 'url(' + image.preview + ')' }">

    </div>

  </div>
  <div class="btn-container">
    <button type="button" class="submit-btn" v-on:click="uploadImages">Submit Images</button>
  </div>
  <br><br>
  <div class="block">
    <h3>Uploaded images</h3>
    <div class="files-preview">


    </div>
  </div>
</div>

I’m trying to display a preview of some images when a user selects them before he uploads them, but I’m new to vue and I can’t figure out how to apply the background image url style to each div (it must be with background image), here is my UploadComponent so far:

<template>
    <div class="content">
            
            <h1>{{ title }}</h1>
            <div class="btn-container">
                <label class="button" for="images">Select Images</label>
                <input type="file" id="images" name="images[]" multiple="multiple" @change="selectFiles"/>
                
            </div>
            <br><br>
            <div class="block">
                <h3>Selected images</h3>
                

                <div v-for="image in images" v-bind:key="image" class="image-result" v-bind:style="{ backgroundImage: 'url(' + image + ')' }">

                </div>

            </div>
            <div class="btn-container">
                <button type="button" class="submit-btn" v-on:click="uploadImages">Submit Images</button>
            </div>
            <br><br>
            <div class="block">
                <h3>Uploaded images</h3>
                <div class="files-preview">
                    
                    
                </div>
            </div>
    </div>
  
</template>

<script>
import axios from 'axios';

export default {
  name: 'UploadComponent',
  data: () => ({
      title: 'Select your images first and then, submit them.',
      images: []
  }),
  methods:{
      selectFiles(event){
        var selectedFiles = event.target.files;
        var i = 0;

        for (i = 0; i < selectedFiles.length; i++){
            this.images.push(selectedFiles[i]);
        }

        for (i = 0; i < this.images.length; i++){
            let reader = new FileReader(); //instantiate a new file reader
            reader.addEventListener('load', function(){
                
                console.log(this);
            }.bind(this), false);  //add event listener

            reader.readAsDataURL(this.images[i]);
        }
      },
      uploadImages(){
        const data = new FormData();
        data.append('photos', this.images);

        axios.post('url')
                .then(res => {
                    console.log(res);
                });
      }
  }
}
</script>

The divs are being created but I’m missing the part where I assign the background image, how can I solve this?