Yahoo Inc. (YHOO) is expanding a test of its mobile telephone advertising network to include more visually appealing messages.

The addition of graphical ads, announced Tuesday, comes a month after Yahoo began delivering short, written messages to mobile phones, relying on some of the same algorithms used by its Internet search engine. That, too, is being done on an experimental basis for now. The first graphical ads are set to go live later this week.

The Sunnyvale-based company is betting more advertisers will be trying to reach the mobile phone market as more people rely on increasingly sophisticated handsets to surf the Web, check their e-mail and download media content.

Internet search leader Google Inc.(GOOG), which runs the Web's largest ad network, also is gearing up for an aggressive push into the mobile phone market.

While Yahoo has gotten a head start in the mobile advertising market, investors have been more disappointed with the company's inability to keep pace with Google on the Internet.

Through the first nine months of this year, Google had reeled in a profit of $2.05 billion — well ahead of Yahoo's net income of $1.21 billion during the same period. The disparity has contributed to a more than 30 percent decline in Yahoo's stock price so far this year while Google's market value has climbed by about 15 percent.