Okay without a lot of code research. The issue appears to be in the version 7.56p2 not the current version 7.5.7.
I don't see a option to turn it off for just topic lists in either versions.
But I enabled the group image for both 7.56p2 and 7.5.7, only 7.56p2 shows the group image in the topic list.
I can only assume it was a undocumented change in the new version for housekeeping reasons to streamline the bandwidth usage.
So bottom line is I assume it could be modified in the code for your version. Or upgrade to the current version.
