I could be incorrect but I was under the impression that rasters in a Raster Catalog are rows in a table (in a FGDB), stored in the Raster Column. That is, there is no file type like .png, .tif, etc... and searching for such files will turn up nothing because you "load" the cataog with those rasters, but the FGDB does not maintain them as individual .tif/.png files.
Edit: if the rasters in the Raster Catalog are not "managed", then the catalog simply maintains links to individual rasters (.tif, .png's) on disk. In this case, you could specify the location/folder on disk, search for and replace the file extensions for each of the rasters. Is this what you need to to do?
james