You look at the at the family of axlGrpDrw APIs. There is no skill code that will translate an existing bmp to line draws there are commerical/shareware programs that will attempt to do it.You would have to take this type of output and convert it to axlGrpDrw APIs
You can also pass a full path of a bmp file to axlGrpDrwBitmap to avoid BMPPATH issues.
In <cdsroot>/share/pcb/examples/skill/basic/* has a simple example.
You can also try converting a bitmap file to encoded Ascii (look at uuencode/decode as an example). Embed the encoded Ascii in your skill file by assigning it to a variable. When the code is executed, it would save the encoded ascii to a temp file, decode it back to binary and load it via axlGrpDrwBitmap.