I was just trying to solve this issue myself... what worked for me was to export a 512x512px bump texture as a .dds from an existing shoe.rx3. I then opened it in photoshop and opened the bump texture I wanted to use as a replacement (but only had in .png format), I copied and pasted the new bmp texture and saved the .dds. At this point a popup comes up asking a number of questions regarding how to format the .dds. Just make sure that you save it as NVTT. Once you've done this, you can import the newly edited .dds into the boot of your choosing. Let me know if I've been unclear. It's a shame that we cannot just import png files in these situations. Hopefully this'll be ironed out in future releases.