I see what you mean, but grouping files in the panel would not help you in your templates. Or only if grouping them would add a new field to your meta files, like it is done when you sort images in the panel. As long as such a feature does not exist, you can add a field to your image meta files, e.g. a field group_by and then filter your images by that field.
I think in your case it is more useful to use filterBy() instead of groupBy().
You image meta files should have a select or checkbox field, let’s say “group” with options “slider”, “featured” and “footer”. Then you can filter in your template by that field, e.g.