For best results, widen your window so images appear side-by-side. Clicking on a cluster name will bring up a set of four images, with optical on the left, X-ray on the right. Each cluster is shown at two spatial scales: images are either 0.5 Mpc or 1 Mpc square (assuming Ho = 75 km/s/Mpc and no cosmological corrections). For each cluster, all four images are centered on the same point in the sky, with standard orientation: North is up, East to the left. Some cluster names have asterisks -- these are links to special notes about individual clusters.
The optical light is mostly from stars in the cluster galaxies (which appear as extended objects), but there are also plenty of foreground stars in our own galaxy which appear as points. The X-ray emission is mostly from diffuse hot gas spread throughout the clusters; individual galaxies can also give off lots of X-ray emission, either from hot gas or from stars in binary systems dumping gas onto white dwarves, neutron stars, or black holes. Most of the X-ray point sources you will see are from distant "active" galactic nuclei, which are also powered by gas falling into black holes.
The images are 512x512 pixels on a side, scaled down to 280x280 (for PC/Macs) or 400x400 (for workstations), depending on your preference. If you click on an individual image in the correct way, you can see it at full resolution.
Shortly, the image quality will be improved and more clusters will be added . . .