Raster images display rough and pixelated in ArcScene, while they display fine in ArcMap.
Layer property adjustments are needed.
If the image is continuous tone, such as an aerial photo or satellite image, set the resample method to 'Bilinear' on the raster layer's Display tab.
Increase the raster quality value via the slider on the layer's Rendering tab. The amount you can increase the quality is dependent on the resources of the computer (machine ram, graphics card texture memory, and so forth). Therefore, don't just move the slider all the way up in one shot. Try it one or two steps at a time.