Thanks for the reply mediawerk but that’s not the case. Initially I had all the images display normally before making them optional using ACF. There were no commas then. The commas have appeared only since using ACF to display the images and I’ve checked all code and values for stray commas – there are none.
It seems to me as though ACF is more geared towards displaying text, and somehow separates each text value with a comma automatically. This is useful but not when the values are images rather than text.
