Authors usually are 'consulted' about the final title and cover image of their books, but that doesn't mean they have much power if they don't like them.
That was situation for Mark Penn. The publisher wanted to call his book "Minitrends" and have an image of a snowball on the cover. Penn happens to be the head of an international PR agency and an expert pollster, so he convened a focus group of about 200 book readers and asked their opinions.
Turns out they didn't like the snowball and they preferred the title, "Microtrends." Confronted with the evidence, the publisher went along with the changes (now the cover features a magnifying glass).
You can influence the publisher's marketing people if you present them with facts, rather than just pitting your opinion against theirs.
By the way, the marketing experts at Pearson decided that the cover of my current book, "Focus: the power of targeted thinking," should feature the world "Focus" in large type against a red/orange background with a sunburst pattern. It seems to be working. The most recent good news: it's going to be the Business Book of the Month for December at a major UK book store chain (I don't know whether I'm allowed to reveal yet which one--I suppose if I revealed it, it would be water under the stone, er, under the bridge...). If you're not familiar with the book, here's my mini-sales pitch: