Hi Kyle,
If I understand what you are asking - you would like to specify just a link and have the image stored at some other site displayed on a Wikitree page. The equivalent to using html <img src="http://www.example.com/picture.jpg">
My opinion based on this interpretation of what you are asking is:
1) It is not 'ideal' to just display another site's content, even if the material is public domain. a) There is the issue of broken links if content is moved or renamed - perhaps less of a concern with a site like Wikimedia, but if you allow links they can be made to all websites. b) There is the issue of bandwidth usage for the other sites. c) It is also unfair if the site hosting the content and providing the bandwidth displays ads or seeks donations to pay their bills as those ads and sonation requests will not be seen. d) It may or not be legal depending on the specific rules of the country where the linked content is and the external sites terms of service. Some jurisdictions may consider a photo of a painting to be subject to copyright even if the painting itself is not (similar to a derivative work)
2) By having users upload images themselves they have 'greater' personal responsibility to ensure material IS free from copyright. People are more likely to think about the legality and rights when they have to download and upload content themselves.
3) Privacy - when a users computer 'pulls' content from a third-party site their personal information may be collected and used subject to that other sites privacy policy without the user being aware of - and agreeing to - that usage. That policy may or may not be the same as Wikitree's.
