round_grill.tga is a regression test supplied by Doug Rogers, associated
with https://github.com/OpenImageIO/oiio/issues/2900 and relted to Targa
images with thumbnail records that that have 0x0 size to indicate no
thumbnail.

For the other images,

    CBW8.TGA  CTC16.TGA  CTC32.TGA  UBW8.TGA  UTC16.TGA  UTC32.TGA
    CCM8.TGA  CTC24.TGA  UCM8.TGA   UTC24.TGA

These files were originally part Truevision's "TGAUTILS" distribution
containing the Targa file specification, code utilities, and these sample
images. It was available to all, free of charge, from their public FTP
server: ftp://ftp.truevision.com/pub/TGA.File.Format.Spec/PC.Version/

The source code files in that directory was clearly marked with a copyright
notice:

    ** Copyright (c) 1989, 1990
    ** Truevision, Inc.
    ** All Rights Reserverd

Presumably that same copyright applies to the sample images as well, though
they were not specifically labeled in any way. While presented in an "open source" way (free, openly available for downloaded, and clearly intended
for wide use), they lacked any explicit open source license as we would
expect now (remember, they date from the 1980s).

This FTP archive doesn't exist any more. Truevision doesn't exist any more,
it was bought by RasterOps. RasterOps doesn't exist any more, it was bought
by Pinnacle. Pinnacle was sold to Avid, but then Avid sold (some? all?)
Pinnacle products to Corel.

It's not clear who at this point owns these files or what the licensing
terms are. Probably none of these companies even know themselves, and likely
don't care.

So I'm going to take the chance that if there is anything actually
protectable in these images (they are just small images of vertical
stripes), and they belong to any company still in existance, that they
probably understand them to be what we would now consider open source. But
if not, and an owner can be identified and contacts me, I will be happy to
remove these files if that is their desire.
