First you acknowledge the problem, then you figure out ways to change the perception. What can Dion do to improve his image, as it relates to the leadership question?
I saw the first hint of a strategy when Dion rallied MP's in caucus, with the requisite media access. An orchestrated event, it was important to let a suspicious press see Dion, center-stage, leading the charge. This little photo-op won't really resonate outside of Ottawa, but it could have some impact on the people that give us the information. A small step towards credibility, but the kind of thinking that could payoff in the long run.
I would plan a Dion trip to Afghanistan. Frame it as a fact-finding mission, a hands on attempt to better understand the situation. Meet with Afghan officials, Canadian military, reconstruction teams, humanitarian officials. Such a trip has a practical application, but politically speaking this trip could do wonders for Dion's image.
Speaking of travel, Dion should meet with some European allies, to discuss environmental issues. I'm sure Dion could get a meeting with Chirac, EU officials. Lead the charge on the revised Clean Air Act, let others know that the Conservatives don't speak for the majority of Canadians. Undermine Harper, elevate yourself, on a stage that speaks leadership.
In terms of marketing Dion, any ad campaign should put the leadership question at the heart. Cobble together any footage or stills that show Dion speaking to a large audience. Dion on the stage, others watch intently, as a leader speaks. Remind people that Dion has led, whether it be the Climate Control conference, during the last referendum, or even a simple rally. I don't think you can underestimate the power of imagery.
Any other ideas?