Hi richard,
I do not exactly know what is Image pooling But since they are concatenating all the layers obtained by spatial pyramid pooling later so I am expecting It is a pooling operation which is not changing the dimensions. I was guessing It will decrease the number of channels(I might be wrong). I have gone through other architecture DeepLabV3+ Encoder-Decoder with Atrous Separable Convolution for semantic Image segmentation. Even there no mention about Image Pooling