It depends on how you read the image. Typically,  torch dataset classes read the image and store in the range 0-1, hence using mean 0.5, std 0.5 makes the data in the range -1, 1. If RGB image from your dataset is with range 0-255, you can either divide it by 255 to keep it in consensus with typical torch approach or use mean 128, std 127.
About using a specific value 172, you may need to consider a larger use case rather than just the dataset that you have. If it’s RGB, ofcourse there could be test images with range more than 172.
Hope it helps.