Output images from the Online DSS at ESO/ST-ECF are delivered in two formats: FITS and GIF. Furthermore you can choose to display the images directly instead of first saving them to disk.
Image sizes for FITS, gzipped FITS and GIF are 260kB, 110kB and 70 kB respectively for a field of 10*10 arc minutes. There's a limit of around 4 MB for the largest image to be delivered. Images from the DSS2 are bigger, because the pixel size is smaller.
For image display, we use MIME (Multi Purpose Internet Mail Extensions) types to launch an appropriate application at your site. You might have to configure the global or personal .mailcap file at your site to map the application/x-fits type to a FITS image viewer. FITS is the standard astronomical data interchange format. Possible viewers are SkyCat , xv, SAOimage or FITSview (MS-Windows).
The ESO SkyCat tool takes advantage of the world coordinates (WCS) keywords present in our DSS images and displays celestial coordinates at each cursor position. If you want to use this feature you need to make sure that SkyCat is installed in your system.
Then, to use SkyCAT as a viewer for FITS files please add the following line to your $HOME/.mailcap file: application/x-fits; skycat %s