Originally Posted by MrB
Because I've not known what size image would look best on the page, I've resized the originals in different versions and named them as a percentage of the original. Would I be right in thinking that that's the cause of the problem in 4 and that I just need to rename them?
Right, you can't use a naked "%" sign in URIs, because it has a special meaning. You don't need to rename the file, but you should write the filenames in the HTML code as '../Images/TRW50%25
.JPG' instead (%25 means the "%" character, see percent-encoding
It means you are referring to some file that is not listed in the OPF. Since you are using Sigil, it creates the OPF automatically, so you are referring to a file that has not been "added" to Sigil. It may be just a consequence of the issue with the "%" character. If it isn't, take note of which filename is given in the message and add it to Sigil.