The OVR file format is according to Google a ArcGIS pyramid file. They now also use it if you import a geo-referenced image. Until version 5.30 they tiled images into lots of small ones, having multiple sub-folders in your project with thousands of small JPG's. Now it's just one big file.
They might have introduced it to make zooming in and out quicker, or to save memory. This way TBC doesn't have to keep the whole image in the memory all the time but is only loading the tiles and zoom levels it just needs. Orthophotos from Propeller i.e. can easily be 10 GB in memory.