Yes, getTargetImageName replaces getTargetName for closeup images. For thumbnails there is now another method called getTargetThumbName (jAlbum now supports different file formats for thumbnails and closeup images).
Sorry for causing this trouble,but why the need to call getTargetName directly? Aren't the variables provided enough?
I've released 18.3.1 now as a core update. It has the name getTargetImageName reverted to getTargetName. Once our lovely admins and testers have verified that this version works (I broke URL encoding in 18.3), I'll auto-update jAlbum 18.3 to 18.3.1.