OK, I tidied up the folder (mainly by deleting the debris
) and put some "skeletal" icons .png files (@WT - what would be the proper descriptive name for .these files?) in there.
I find that I need a few sets to match the different themes (for light /dark background, colour co-ordination etc), but the job is a lot easier. So I have sets of file in different sub-folders and copy the appropriate set into the toolbar_icons folder.
Thank you for the guidance.
Top-tip: as Tod said, give each of your own .png files a unique name so that the intended image is used rather than one built into the theme.