Assalamoalaikum,
From the attached document, the specs are very clear, and as there is no need to make any code changes in the header files, this will be a simple C code that takes up parameters from command line, calls the predefined functions to get the pixels in memory and then perform the simple addition (of unsigned integer) with clamping of value between 0 to 255 into each pixel's existing values. The only error checks needed are to ensure that parameters are passed correctly and that clamping is done properly.
Obviously I will be needing the header files that you have created for BMP and PPM, and documentation on calling their functions.
Will be looking forward to discussing more on the project with you.
Regards.