Over at Destiny Wiki, we have an optional infobox for use on user pages. When I converted this to a portable infobox however I found that the external images used by users would not display. I managed to make a work around in which the image appears inside the title tag, however it will not work for mobile users (as expected).
Would it be possible for external image support to be implemented?
That being said, you can ask staff to enable a "shared image repository" that will allow your wiki access to images stored in one (?) other wikia. You can also ask for access to wikimedia commons.
Ideally, external image support would be limited to only user pages.
Portable infoboxes were deliberately designed to use local images for various reasons. Your workaround will not work because the <title> tag strips any images and leaves only text. A title is made up of text not an image anyway. You could try using a regular <data> or <navigation> tag for that.
If that still doesn't work, I'd recommend suggesting your use-case to staff (Special:Contact).