I do not have any image in the file. It is just the sensor data I am getting from the ADC/FPGA combination.
Then it will not be a recognizable graphics format such as GIF, PICT, PNG, BMP, TIFF, etc.
You will need to learn how to create the image onscreen. You'll need to plot each pixel, draw each shape, etc.
I do not know C programming. The commands are doubtless different from BASIC commands such as PLOT, LINE, BOX, CIRCLE. In addition you'll need to set up a window size, type, color mode, etc. You need to scale your data to fit the size of your screen. Etc.
The necessary commands may or may not be in the version of C++ used in your module.
There may or may not be the command which records everything that you draw to screen...
and a command which converts the drawn image into a single file as a GIF (or PICT, etc.)...
and a command which saves it to disk as a file which is usable by other applications.