When it comes to mobile marketing, two channels have gotten most of the attention: text messaging, because almost every phone in the nation can be reached via SMS, and for little cost; and specialized downloadable apps for smartphones, because they create the most buzz, tap into a relatively affluent consumer, and may—just may—earn a little revenue if people are willing to pay something for them in iTunes.

But there are other ways to get an explicit marketing message out to handsets, and research firm eMarketer predicts that one of them—ads displayed in various places on mobile content, including content pages, search results and as independent ads within text messages—will draw increasingly large budgets over the coming four years.

According to the new report, “Mobile Advertising and Marketing: Change Is in the Air,” spending on all mobile advertising in the U.S. will grow to $1.56 billion by 2013—almost five times the $320 million spent last year and about triple the $416 million forecast to be spent on mobile advertising this year.

And looking at the channels those budgets will be spent in, eMarketer has determined that while 22% of mobile ad spending today goes to display ads, that proportion will increase to 35% by 2013. Paid search ads in mobile will grow even more as a percentage of total spending, from 18% now to 37% for years from today.

Meanwhile the proportion of mobile marketing money going to messaging campaigns – currently 60% of all spending on mobile advertising—will drop to only 28% by 2013, eMarketer forecasts.

In other words, the $228.8 million spent on SMS campaigns in the U.S. this year will grow to $436.8 million by 2013, for an annual 2008-2013 growth rate of 17.9%. But display spending in mobile will grow 50.6% annually for the same period, from $104 million this year to $546 million in 2013. And mobile search ads will increase at a CAGR of 58.6%, jumping from $83.2 million this year to $577.2 million in 2013.

Several factors account for an increasing focus on mobile display ads, according to Noah Elkin, senior analyst at eMarketer and the author of the report. Primary among these is a growing reliance on the mobile Internet. EMarketer’s forecast predicts that 36% of mobile subscribers will access the Web over their handsets at least once a month by 2013, up from 26% this year. And more mobile web surfing will mean more exposure to display ads on Web pages.

The other driver for mobile display advertising is the rise of smartphone apps—and specifically of apps designed to be free to the user by virtue of ad support. As more people use apps, more people will also see in-app display ads on their smartphones.

Finally, mobile display ads should get a boost from a widening array of engaging formats and inventory options. Banner ads are the predominant display format in mobile right now. But as mobile screen become larger, as touchscreens proliferate and as more users get the option of downloading applications to their phones, marketers will be able to make better use of more interesting ad types, including interstitials, page takeovers, rich media and even video.

From the advertiser’s perspective, mobile display ads have an attraction because they tend to encounter less clutter than display ads on a PC browser. The smaller screen size of handsets means that many mobile pages that make room for display ads do so for only one ad at a time.

And several mobile ad networks are reporting ad-clickthough rates that average 10 times higher than those on PC-viewed pages. A number of mobile ad networks have partnered with wireless carriers and are thus able to use some subscriber information to target the ads they display more closely, perhaps accounting for the higher conversion rates than on desktops.

Mobile search will be as important to navigating the mobile Web as PC-based search is now, and thus, eMarketer says, will provide as many opportunities and reasons to get a keyword-based search ad in front of users who are looking for a specific category of product or service from a handset.

And while the percentage of mobile ad spending going to search now is small—18%, per eMarketer’s estimate—the potential audiences are already large. Researcher comScore determined that 11% of mobile users accessed some kind of Web search from March through May of this year. Nielsen found that search engines had 17.8 million users in March. And The Kelsey Group anticipates that U.S. mobile search users will grow to 56.2 million by 2013.

“while many marketers have been reluctant to embrace a channel that lacks proven use cases, an increasing number have begun to realize the ‘additive effect’ of mobile on their other advertising and marketing efforts,” said Elkin in a release. “As smartphones proliferate and marketers move beyond experimentation, budgets will steadily increase.”