Among the not-exactly-documented features of WWT is the ability to load your own images to view as panoramas.
For an example, consider the Summit Panorama from the NASA/JPL Mars Exploration Rover Spirit.
Click on the image (or right click and 'Save Target As') to get a 4096 x 2048 jpeg (600k), or click here for a 8192 x 4096 version (2Mb).
Now, using the "Look At" menu in the bottom left of WWT, choose Panoramas->Browse

This pops up a dialog box where you can enter the file you want to upload. Try the 4096 x 2048 version before you try the 8192 x 4096 version. (And yes, you can also load image formats other than jpeg.)
This should, after a few seconds, display the image in the WWT display window. There will be distortion at the 'poles' since we are using equirectangular projection instead of WWT's usual TOAST projection.
This post also applies to equirectangular all-sky maps in equirectangular format.