You can. Anybody can. Just create a project and start uploading.
There are three kinds of projects in Central:
Regardless of whether a project is public, protected, or private, only owners and members of a project can alter the data. Collaborators can access the data but not alter it.
Right now, you can stick as much in it as you want. Eventually, we'll run out of storage space and then we'll need a new strategy. Until then, have fun.
DICOM. Eventually we'll support other formats, but stick to DICOM for now.
Right now, non-image data is limited to some basic demographics (age, gender). Eventually we'll add tools for storing a range of clinical, genetic, behavioral, and other data.
Not yet, but soon.