ASIC just means it's an application specific IC (= chip), meaning it's fabbed for that specific product (like in this case the Raspberry Pi). A functional block like a JPEG codec contained therein is not an ASIC. Quoth wikipedia:
"Modern ASICs often include entire microprocessors, memory blocks including ROM, RAM, EEPROM, flash memory and other large building blocks. Such an ASIC is often termed a SoC (system-on-chip)."
What you're describing in the JPEG codec might be termed a fixed function IP block in semiconductor design terminology.
An application-specific integrated circuit (ASIC /ˈeɪsɪk/) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficiency video codec.