So my question here is, where can I find this SIP image for this specific phone?
A valid Service Contract is required in order to download any software. Since this is an 8851 it is, by default, already running SIP software. The main question is whether or not the software version is "compatible" with 3rd party call manager, like Asterisk and such. SIP software for the 8851 can be found HERE.
and are there any instructions somewhere?
Cisco doesn't support integrating the phones to non-Cisco call managers, however, search for "Asterisk" in this forum and there should be a few threads available.
For a start, post the SEPmacaddress.cnf.xml file and I'll have a look.