I think there are two issues here, and I'm not sure which specific one being asked for by the OP.
First, with Raspbian images produced by the Raspberry Pi Foundation, you'll need to ask them to provide a list of changes that may have gone into producing the image. However, I don't think that this information will ever cover what changes occurred on individual packages that were upgraded from one release of their image to another. Instead, it would probably consist of any changes they made to the configuration scripts of the tools they use to produce an image. Alex, asb in these forums, is probably the person to ask if such change information is available.
With regards to changes that occur in individual packages, as far as I know, Debian doesn't keep track of this information in a centralized manner. Instead, each package has a change log associated with it listing the changes that occurred in that specific package from one version to the next. Since Raspbian is based on Debian, we provide the same information for each package. To view the changes associated with a specific package, the 'apt-listchanges' tool can be used as described here:
http://www.debianadmin.com/show-what-ha ... anges.html
If someone knows of a better way of getting changelog information from Debian, please let us know.