This is how I create images that are full width in desktop devices and mobiles etc.
You just need to use:
max-width: 100%;
height: auto;
Good info here on W3 Schools
<img src="image-address.png"
style="max-width: 100%; margin-bottom:0; max-width:1000px; height: auto;"
alt="relevant description of image" >
Change the “max-width” to whatever you want the max width of the image to be on a desktop device.
For personal reference, this is the exact code I need for my own website with pre-built classes:
<div style= "margin-bottom:20px;" >
<div class="content-item">
<img src="image-url" style="max-width: 100%; margin-bottom:0; max-width:1000px; height: auto;"
alt="relevant description of image" >
</div>
</div>