I am just posting this as a solution for others, as I managed to resolve this after some time.

The problem I had was that I added (FTP) several (circa 20) jpeg images to the canvas gallery folder.

/components/com_comprofiler/images/gallery/canvas

However, on the front end only some of those images were displayed in the canvas gallery select page. All the images were the exact same spec i.e. 700x200 pixels etc. So I was bemused.

In addition, those images that would not be displayed in the gallery would also not upload when selected as an individual image for upload by a user on the front end.

The problem turned out to be due spaces in the image file name. As my site was hosted on a UNIX server it did not like spaces in the file name. As soon as the spaces were removed from the filename the files worked.

I know it should have been obvious but it was not as there is no useful error message. This may be an error that some front end users may also experience, so worth noting.