In the Raster Manager dialog, <Ctrl + click> the names of the additional images. That is, while holding down the <Ctrl> key, click the names of the additional images.

The additional raster filenames highlight in the Raster Manager dialog. If Display Raster Border ( File > Settings > User > Preferences , Raster Manager category's General tab ) drop-down is set to When Selected, the images highlight in the view(s) as well. If you <Ctrl + click> a previously selected image, it is deselected.