NotJohn I thought that filesize seemed small too - but it really is 187KB
The image is a placeholder image until I've designed the cover - it's just a white rectangle with some black text on it. It really is only 18KB
Anyway - I did manage to convert the thing once it was no longer stored in Dropbox (see my post above) with this same cover image so I don't think the file size or the pixel size of the image was the problem. I suspect the fact that Dropbox was overwriting the file while it was being converted made the converter software throw out errors that were not there - at least, that's what I'm guessing.