The data format for the DSP primitive is network-ordered unsigned short integers. BRL-CAD has a couple dozen tools that you can use for converting existing data into that raw format, such as the 'cv' command or the 'bw-d' and 'd-u' commands among other similar tool chains. If you use the cv command, the output format is "nus" for network unsigned shorts.
The DSP takes unsigned short (16-bit) integer data. Our various command-line data converters can help bring data in from pretty much any existing format, including image data, via various processing commands. If the data was in png image format, for example (which is basically 3-channel 8-bit integer data), the data could be prepared with a combination of 'png-pix', 'pix-bw', and 'cv'.
If you type the 'in' command, it will prompt you for each parameter individually and that should help some. For the DSP, the main parameters are the width/length of the input data, width/length/height scaling factors, and whether to smoothly interpolate between cells.