1. Can nabble also be used as a file host for all types of files (e.g. PDF, DJVU etc) upto 5mb in size (I thought that it allowed only image hosting)?
IM not sure about this one , but i think it would be a no
2. Are bulk/batch uploads of images possible (maybe packing all images in a zip/tar and uploading the zip to server or something like this)?
you can zip all your images to 5mb files and then upload them seperately not at one go
3. What image formats are supported (PNG, JPEG?) in image galleries?
both png and jpeg
4. If you dont mind telling, how do you manage to allow to upload unlimited number of files?
Must be the servers having enough space available not sure how they deal with this.
but it is not a image hosting gallery if thats what you are trying to do...