Test other image formats

_From @jakirkham on June 7, 2017 20:19_

Currently we have some tests for TIFFs. However there are no tests for any other image formats. Might be worth covering a couple basic ones. Though don't want to go too far into the weeds. Might be best to see some use cases before digging in.

Edit: Also might need to switch to something like [`imageio`]( https://github.com/imageio/imageio ) for testing so as to generate a large variety of test data.

_Copied from original issue: dask-image/dask-imread#8_