Once loaded, the image may appear grey or transparent. Use the "Transfer Function" widget to map greyscale values to colors. Typically, lower density (air) should be transparent, while high density (metal/bone) should be opaque.
If you are on a device that doesn't support Fritzing or you need a quick look at a file without installing heavy software, several web-based tools are available: .fzz - Programming - Arduino Forum fzz viewer
If you have an .fzz file and want to work with the "pieces" inside: Open the Project File > Open Once loaded, the image may appear grey or transparent
The primary way to view and edit these files is by installing the latest version of Fritzing . lower density (air) should be transparent