If multiple bits per pixel are used to store an image, then the frame buffer…

In raster graphics, a frame buffer's storage scheme is named by how many bits it allocates per pixel. When exactly one bit is stored per pixel, so each pixel can only be on or off (a strictly two-tone image), the frame buffer is called a bitmap. When multiple bits are stored per pixel, encoding shades of grey or colour depth, the frame buffer is called a pixmap (short for pixel map).

The question states that multiple bits per pixel are used to store the image. That is exactly the multi-bit-per-pixel case described above, so the frame buffer here is a pixmap.

• Bit map names the single-bit-per-pixel storage scheme (a strictly two-tone image) — a narrower case than what this item describes, since here more than one bit per pixel is used.

• Binary map is not an established computer-graphics term for frame-buffer storage; it substitutes an invented label for the recognized bitmap / pixmap naming.

• Image map refers to a clickable region defined over an image (used in web/HTML graphics for hotspots), an unrelated concept to how a frame buffer stores pixel bits.