Thanks for letting me know. I've got it on my to do list to test the new gallery in the context of multiple sites and also test it on Mono, so I will look into this soon.
As for descriptions/captions etc I'm afraid this is a case whre there is a tradeoff. The other (older) image gallery included in mojoportal does allow captions and descriptions and its down side is that you must upload each picture one at a time. Its caption, description and file location is stored in the db.
The new image gallery has the convenience that you can upload a folder of images via ftp and just point it at the folder. However its downside is that is only deals with the images. There is no easy way to add caption or description and it has no database component associated with the images.
I'm sure adding that ability is not impossible but probably challenging and its not something I plan to put effort into any time soon mainly because I have a lot of other priorites calling for my effort.
So for the time being, if caption/descriptions are important I would recommend using the other older Image Gallery instead of the new one.
Long term I would rather implement integration with Flickr or other services of that ilk that provide a lot more features.
Hope it helps,