Hi Tom,
Both fsl-image-gui and core-image-base may use some common packages. The packages for compiling are downloaded and stored to downloads folder in the yocto projects main directory (if downloads directory is not present, then downloaded files are stored on the source directory). The packages that are downloaded are stored on to downloads directory irrespective of the image you are compiling(in most cases). So if the file required for 'core-image-base' is already downloaded by the 'fsl-image-gui', then it will be present on downloads directory, and further compilation will not download the file again.