Try:
sed 's=\(<[Ii][Mm][Gg],[0-9,]*>\)prod/=\1special/='

As for the equals sign, my sed man page, as it describes the s command, says "Any character can be used instead of /." Slash wasn't a good choice here since the data also contains a slash. Had I used a slash for the delimiter, I would have had to backslash any slashes that weren't delimiters. Switching delimiters eliminates the need for the backslashes.