Using this tool you can query the current list of Simple Image Access servers for lists of any images that they have for the given region of sky.
You can define the sky region using either an object name lookup, entering the RA and Dec (FK5/J2000) coordinates, making it the same region as the displayed image (Set area from image) or by dragging out a region of the displayed image (Select area...). You can define a height and width for the region, in arcminutes, but not all servers will make use of that information, using just the width. Some servers will also just return a plate containing the region.
Once you have defined the region to check for images press the Query button. That will initiate a series of queries to all the known servers. Successful responses will add a new menu item to the SIAP server results: drop-down menu. Select an item to see the page of images available.
Display or Display in new.
Querying all the servers can take quite a long time and you may have a better idea of those which are relevant to your science goal. To change the list press the View image servers.... Using that you can remove servers from the query list and query a VO registry about additional services not already available, as well as being able to apply a constraint like selecting only servers that have a string in their titles.
Because the VO is an evolving service many servers have been put up for testing only and never return a good set of images. These servers are best just forgotten about and so can be blacklisted. GAIA defines a default set of such servers (see $HOME/.skycat/GaiaSIAPBlacklist) and any servers that fail in a response will be offered up addition to the blacklist.