The most rescent slackware ISO image is 8.1. ISO images are only produced by Slackware for released distributions. Slack 8.1 was released this last summer at the end of June if I recall. For updates look in the slackware-8.1/patches tree.
The slackware-current tree is beta software for the next release. Much of the package will probably work OK, but expects some quirks. The beta is also based on the newer compiler packages that are known to have some problems as well. If you wish to test it out that is one thing, but if for production workstation or server then better stick with the released versions and then update as desired. Slackware does not produce ISO images for slackware-current, others sites may perhaps, but it is easy enough to create your ISO image. The directions are in the isolinux directory of the tree.
BTW: Shouldn't mix the two systems, the packages in slackware-current are NOT built for Slack 8.1.