Obviously the advantages of a project on its own system is complete control. It is also faster because there are less traffic compared ti the throttled bandwidth of free online services.
I have no doubt you can tweak the test site to suit the needs of the group. The following are the results of what I had done so far. I registered and the captcha is a good layer of security. However, once a registered user has successfully logged on, I think filling in human-response tests is no longer necessary thus when I tried to post a comment, I had to type in 4 sets of test and still did not see my comment posted. If the comment is held for approval, IMO that is also unnecessary. I gave up on posting a comment along with rating the test wallpaper. If I made mistakes in reproducing the test, the more reason the post-logon captcha should be removed. The posted image probably just lost a valuable comment.
My access to your test site is fast. I had the software email me another password; the password I was using was invalidated so I had to use the temporary pass to access and reset my password.
I was able to download the only image in the Lightbox to my hard drive (and quite fast as well), modified it and tried to post it back. I do not see any way to upload my modified image. IMO regular members should be permitted to upload/remove their own work.
Overall, I would vote for your site.
@timeth, deviantArt is too slow for me but tolerable. And because of its loading time on my browser (Chrome), the CSS looked weird/the layout looked like a 10-years-ago webpage with white/gray background. Advertisments are also why its free. The time you spent setting it up is truly appreciated. But I think I'll place my vote on Sproggy's.