There's no specific term other than "type specimen" which is simply some sort of design to show off the design of your typeface.
Historically, back when fonts where physical things made out of wood or lead, you'd receive a catalog from the type foundry with their fonts displayed in the book in various ways, though typically in the form of short sentences or words:
The 'small web image' format is just a variation of that, albeit smaller and designed for the screen. It's a style adopted by Wikipedia as well as MyFonts.com
A variation would be a 'glyph chart'. Like a type specimen, it's for showing off the font, but in this case has an additional task of indicating every single glyph contained within the font. The small image is, of course, not ideal for that type of type specimen.