First of all, you can't import sprites like isometric tiles or any sprites or tiles for that matter. This is a planned feature of importing images and is not implemented yet. But it is planned. Right now you can only create with the pre-loaded sprites. Second is that when you buy Kenney Studio you get 2 downloads.
Kenney Studio 0.51 (deprecated) - An older version of Kenney Studio, includes a 3D Axonmetric creator, this version was before the re-brand of Kenney Studio, with a totally different UI. Probably going to drop off the download page in a month.
Kenney Studio NXT 0.2.0- The latest version of Kenney Studio with the new UI, does not include 3D Axonmetric creator. As Kenney said above, this feature will probably not make its way back into Kenney Studio. If you want to create 3D assets, I'd suggest looking into Asset Forge another tool created by the Kenney Group, not out yet though, still in development.
So for now if you want you can create 3D assets you could ( limited features )with the Kenney Studio 0.51 using the 3D Axonmetric creator. The Axonmetric creator, however does let you import 2D assets to attach to the side of the cube. But if your looking to create real, nice looking 3D or Axonmetric assets, I'd just wait till the release of Asset Forge, the old version of Kenney Studio isn't getting anymore updates on the Axonmetric creator, probably going to be dropped off the download page soon. And Kenney Studio NXT isn't getting a Axonmetric creator. Or if you can't wait then just buy a Sketchup subscription( will take some learning though).
Best of luck,