A small image with a generalised, non-customised slogan, used in the same way, is generally just known as a graphics tag, or a snag (because they are 'snagged' or downloaded quickly from an originating website).
The creation of sig tags is a widespread online hobby, regarded as a subsidiary of graphic art.
In fact, a conventional classing system (Novice, Moderate, Intermediate, Semi-Professional, Professional) is sometimes used to rank people on their skills in making sig tags.
Many sig tag websites feature a busy forum and tutorials and users are encouraged to download images.
This may be due to the fact that tagging is often regarded as the lowest skilled form of computer graphics and the least profitable on many large forums.