This directory contains the images used to assemble dialogues. Each group of images is named by the part before the first dash, which is a style referenced by name in the Wesnoth UI code.
The 'transparent' images have some alpha transparency; they look like smoked-glass overlays on the terrain or whatever else is underneath them. Each transparent style has a suffix which is its opacity percentage (100-alpha). The 'opaque' images are geometrically the same, but opaque. They could be called 'translucent100', but aren't for reasons that are basically historical.
Note that the translucent and opaque styles differ only in the background images, sharing borders. This uses a little more space but simplifies the code.
The directory also contains some background images of different opacity levels, just so they're handy for tuning. They cost less than 0.5K each to keep around.