General:
* Miniature
* Thumbnail portrait
* Tiny portrait
* Pocket portrait
* Small-scale portrait
Specific to style:
* Cameo (if it's a carved or sculpted portrait)
* Vignette (if it's a portrait with a faded or softened edge)
* Enamel portrait (if it's painted on enamel)
By size:
* Miniature (typically very small)
* Keepsake (implies a small size and sentimental value)
* Charm (if it's a small portrait worn as jewelry)
By subject:
* Self-portrait (if it's a portrait of the artist)
* Child portrait (if it's a portrait of a child)
The best word to use will depend on the specific context and the style of the portrait.