Well, this probably isn't very practical for you, but I'll throw it out there anyways...
Over the last while I've been doing a lot of work on Samsung's Family Story product. It's basically a mini-social network where up to 20 people can communicate very quickly and easily. It hooks in with the rest of the Samsung social network through a Samsung Account (or a mobile phone number, etc.).
Sharing in the network is relatively simple and allows for easy collaboration. Emphasis on easy.
Now, this is unlikely to be all that useful for you as a lot of this stuff hasn't been officially rolled out yet. However, the basic idea is there, and if you can locate anything that allows for similar collaboration, it should fit the bill.
The APIs for most services are usually ok to use for simple things, so you could relatively quickly write up something to pull down pictures, etc.
Flickr is a yahoo company, I'm fairly certain.
I'd be inclined to wonder how complicated (if at all) the web code to upload a picture would be. If it's easy enough to do, and you're not planning to provide guest internet access. Then you could set up a temporary local web server (IIS comes with windows) that had an upload pics to wedding party option on the main page. Then have the WiFi's WAN gateway sent to the local web server so all web requests go directly to it (might require a redirect script, but it makes for a can't miss target.).
It's totally hands off (mosaic display software would just target the server's upload folder), and would it open up the possibility of letting folks browse all the photos taken live without risk of deletion. side benefit being as it's a closed loop there is no risk of data leakage.
And the Flickr API is pretty good. I've used it.
The Facebook API is usable.
You could have people post to specific albums, then download new content from there. But honestly, Box.net or Dropbox would likely be the easiest for everyone.
You could then simply monitor the folder. A LOT less work. You only need to set up the collaboration with people... and that's where all the real work is. It's analogous to the Samsung Account, and how they integrate all the services. (I'm not sure what the official name will be when it's all rolled out as I suspect that the name I'm looking at may just be an internal code name - it's not all that sexy at the moment.)
Both Box.net and Dropbox have mobile applications, so, that solves that. They're both quite good, though I'd recommend Box.net probably. The simplicity of a folder is just so much easier. And they already have excellent sharing options.