remote images instead of upload?

Hi all, I'm very new here. I wrote some very basic ext for joomla! and phpbb3 but I'm not a programmer!!

After looking for a social network software I landed here mainly for 3 reasons:

elgg is open source,  it seems to be fast, it's very simple for the user with a clean interface. I'm trying to customize default template, and it is smooth. I looked into the features and they fit for my project.

The only concern is about file plugin. The only way to limit upload size is in php.ini. No way to assign a kb quota for users and restrict file types. Doing the most basic maths this will full my vps space in a handful of days...

I don't know if it can be a foolish idea, but I would like to see an image gallery using remote images. So the users can just host them in flickr, picasa, etc. and link them. It is actually possible in pages, blog even here in this message... 

I have no idea if gd can handle such remote image to create thumbnails (even I saw something similar in some popular sites, not to mention google does)

Anyone can suggest me if starting to mod file plugin could be any easy - hard - impossible?  just in order to replace the file upload field with the url of a remote img.