The web development company, which started in 2006, has received investment from Horizons Ventures, NorthZone Ventures and Atomico.
Halpin left Expedia, where he had been European president since 2006, at the end of December last year.
Alex Willcock, Imagini's founder and chief executive, said: "We are off to a strong start this year and we are thrilled that Dermot has agreed to help spearhead our ambitious growth plans."
Imagini has developed technology that gives advertisers, publishers and agencies detailed profile information on their online target audiences. The technology works by asking client web users to respond to picture-based quizzes using Imagini's VisualDNA technology.
The bespoke quizzes can either run on a company's website or be embedded into Flash, making them easy to distribute via online ad and affiliate networks.
Halpin said: "I believe that Imagini's technology cuts straight to the heart of the most important aspect of any communication today -- understanding your target audience.
"And we do it in a way that is both fun and easy for people to use, and creates genuine competitive advantage for our customers. I believe the prospects for this company are outstanding."
Halpin joined Expedia in the UK as managing director in 2001, a role he filled for three-and-a-half years. In early 2006, he moved to head up the firm's European operations.