Display an image from an external database
Views fields have an option to overwrite the output of the field. You can use this to add any html you want, using twig. You'll have all fields rendered before your field available as twig variables.
In your case, since you know that you're getting an image url from your external database, you can rewrite the output of your field using twig. For example:
&lt;img src='{{ my_image_url }}' alt='{{ &quot;Alt text for my image&quot;|t }}'&gt;

To access the Rewrite Results option, click Edit on your field in the Views UI. Near the bottom of the dialog, you'll see several collapsed fieldsets. Open the Rewrite Results fieldset and check the box for Override the output of this field with custom text. You'll get a text area and another fieldset, Replacement Patterns, below it that lists all variables available for your rewrite. Find your field name for your image url and write your output similar to the snippet above.