and it is possible that the host has some caching going on like cloudfare that takes some time to clear.
you replace the images with images of the same file type and file name by just uploading to let them overwrite the ones that are there.
if you are changing from one file type to another then you have to just upload the new images to the folder and then edit the relevant css file( s) to change the file type extension. most of the time that will be in the colours.css file.
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OK, so what's the speed of dark?