An image from ds9 comes back as an HDUList (as expected for the output of `fits.open(...)`:
```
In [21]: data = receive()

In [22]: data
Out[22]: [<astropy.io.fits.hdu.image.PrimaryHDU at 0x106507950>]

In [23]: type(data)
Out[23]: astropy.io.fits.hdu.hdulist.HDUList
```