Should I normalise image pixel wise for pretrained VGG16 model
It all depends on how the original pretrained model was trained. If it was trained with normalized data, you should also normalize your data before giving it to the model. Otherwise, the input data distribution will not match what the network was trained with, and then you probably won't obtain good results.
VGG16 pretrained weights expect normalized data as input, so the answer is yes, you should normalize the data (subtracting the mean and dividing by the standard deviation).
If your image domain is similar to ImageNet, you may use the same mean and standard deviation statistics. If your images are from a very different domain, you should compute your own statistics (source).