In a recently published video clip of W. G. Sebald reading at 92nd Street Y in New York, he explains why he puts pictures in his books:

I put these pictures in in the first instance for my own satisfaction. But as I kept doing this I realised that conceivably they had other functions in the text which one could acknowledge. They hold up the flow of the discourse and, as one knows, as a reader one tends to go down this negative gradient with the book that one reads towards the end. Books have almost by definition an apocalyptic structure. And it is as well therefore to put weirs in here and there who hold up the inevitable calamity. 29:19-30:08