I understand your issue, but if you make many changes to buttons, doing things the way you mention could result in your Gallery > Buttons folder getting clogged up with a lot of redundant files.
Captivate caches the Image buttons originally inserted from the Gallery > Buttons folder inside the CPTX file. So if you update the files in the gallery but don't change the filenames, Captivate doesn't show you the new buttons IF YOU ARE STILL LOOKING FROM THE ORIGINAL CPTX.
However, if you were to open up a brand new CPTX project and then look to insert buttons from the Gallery, you WOULD see the new version of those buttons.
It's an annoying quirk in Captivate's behaviour that nobody has mentioned much lately because most people seem to be using Smart Shape Buttons instead nowadays.