Can I apply an image transformation to an arbitrary image (non-Assets field)? Curious if I'm able to use the resizer on a random, sourced image, not coming from a field anywhere.

So I would just be passing a string to the path of the image.
While transforms are an asset-only feature, you can get at all of the underlying image manipulation features pretty easily using the ImagesService.

$image = craft()-&gt;images-&gt;loadImage($imageSource);
$image-&gt;setQuality($quality);
$image-&gt;resize($transform-&gt;width, $transform-&gt;height);
$image-&gt;saveAs($targetFile);