I don't have Premiere, but I do use image sequences all the time. The render frame rate depends on the render settings. So even though iClone internally uses 60 fps, you will get half the number of frames if you render out at 30 fps. In Premiere, check how it interprets the image sequence. It should see it as 30 fps, in which case it would play as a 30 fps video. I use Vegas Pro and I have never had any issues with image sequences from iClone or Iray for that matter.