I know that if I want to have a html field that let's the user upload a local file I can use
<input type="file"></input>
However, let's say I am guaranteed that all images will be at a valid absolute URL to an image. How do I let the user specify that? What input type do I use?
You may have to use:
<input type="text"></input>
HTML5 has url input types:
<input type="url"></input>
Consult the MDN on inputs for more inforation.
If the user is just specifying a URL at which you can find the image, then all they're really entering is text. There's no input element that's specific for a URL.
<input type="text" />
What you do with that text is another story entirely. There's no way for the HTML to validate that it's a URL for an image, a valid URL, etc. That's all stuff you'd have to do in code.