failed for running just an image

[pascal-maker]

I try to run the command ns-process-data images --data=/Users/pascal/Desktop/tumorimages --output-dir=/Users/pascal/Desktop/processed_tumor, nerfstudio is installed. and this is the output is No usable images in the data folder. process_data_utils.py:411. The images are in the same directory as the code and are just some png files.