First, the string isn’t random. It’s a cache key that includes a hash and image parameters.
Second, according to John Mueller of Google, if you already provide an alt attribute the filename makes little to no difference. I’ve written about this before:
https://discourse.gohugo.io/t/resizing-images-and-renaming-them/46552/6
And I’m sure your alt attribute is excellent in order to comply with accessibility guidelines.
Third, if you want to rename the image, use the resources.Copy function.