That is really the way it is supposed to work.
Text is allowed to reflow, so a
paragraph fills the line, as wide as it is.
A Picture, for all practical purposes, is a Fixed object. (IMHO Stretching looks bad. Stretching over 300% wide
) . The cover is typically a single page, Just long enough to hold the picture. You might try setting the justification for that Paragraph to Left, but not all software supports the setting (It hurts nothing to try
)