Believe it or not, Dropbox can be used for something like that. I'm currently using Dropbox for album art for all 3 of my stations. Then my website grabs those images from Dropbox to display them on my sites.
When sharing files with multiple people, everyone can have access to the files locally by installing the Dropbox app onto their computer and adding the Dropbox folder.
The key to this is, you must use the public folder in Dropbox and each folder will have a unique path to it.
In RadioDJ, copy your album art to the Dropbox folder for it and in Options, options point RadioDJ to the Dropbox folder. Doing that automates the process. No more uploading album art via ftp. Then in your php/html files point the art URL to the unique folder in Dropbox.
By the way, Dropbox gives you 10gb of data transfer per day and they even allow using Dropbox for your website's images. They actually encourage it.
As for audio files, in RadioDJ you can add the files as VDF so if the file names are the same everyone will get the latest. Remember, when using the desktop app, the files are actually downloaded to your local drive. So there won't be a data rate problem.
The only drawback is how often are the files updated/synced from Dropbox to the user folders on their PC.