Most of the images on AstroPix are available for most public uses, but the details vary by institution. The AstroPix site only aggregates images offered by its partner institutions into a unified archive, but as such this site has no universal usage policy.
You will find the image use policy for each individual image at the bottom of the left column under the caption text, credit, and source link. This may be a simple statement (e.g. "Public Domain") or it could link to a more detailed policy of the responsible institution.