SAN FRANCISCO » Twitter has gone visual.

The social network, which has been built around 140-character snippets of text since its founding in 2006, has added photo and video previews to the feed of items that users see when they log onto the service from the Web or mobile applications. In the past, Twitter users had to click on a link to see a photo or video.

The change, which helps Twitter catch up to recent moves by rivals like Facebook to showcase photos and videos more prominently, could help increase the use of Twitter as the company prepares to sell stock to the public for the first time in an offering expected to occur next week.

The addition could also help the company sell more ads with visual elements.

Robert Peck, an Internet analyst with SunTrust Robinson Humphrey, said that the tweak to Twitter’s look addresses a concern he has heard from potential buyers of Twitter’s stock. "It was all text, for the most part. There was no multimedia," he said. "People thought Twitter was behind."

Twitter has traditionally been very reluctant to tinker with its message feed, which it calls the timeline, because it has wanted to keep its display of tweets as streamlined as possible.

The turn toward the visual is the biggest change to Twitter’s interface since it was overhauled in 2011, although the company has recently introduced other tweaks, including a blue line that groups related messages so that users can more easily follow a conversation.

With Tuesday’s change, tweets will still show up in chronological order, with the most recent first. But the tweets that contain photos uploaded to Twitter or six-second videos from Vine, a video creation service owned by Twitter, will automatically preview those images.

"Starting today, timelines on Twitter will be more visual and more engaging: previews of Twitter photos and videos from Vine will be front and center in tweets," Michael Sippey, Twitter’s vice president for product, wrote in a blog post Tuesday. "To see more of the photo or play the video, just tap."

If users embrace the change, Twitter could also add automatic previews of other types of links, like articles and web pages or images and videos from outside sites like Google’s YouTube.

That technology is already used to preview a variety of sites on Twitter’s Discover tab, a little used feature of the service that is designed to help users find new content they might like based on who they follow and topics of which they have expressed interest.

The company is also experimenting with ways to highlight other types of messages, like those about television shows, although no other changes have yet been released to all users.

Although a more visual feed does not directly affect advertisers on Twitter, it does improve the company’s position in the battle for mobile ad dollars.

Instagram, the photo-sharing service owned by Facebook, just began selling visual ads on its service from brands like Adidas and Lexus that are sprinkled into the flow of messages that users see.

Twitter’s principal form of advertising, known as a sponsored tweet, also appears in the stream of messages from users, and advertisers can post sponsored tweets with images in them.

Industry research shows that users are far more likely to click on an ad with a photo in it. Since Twitter only gets paid by the advertiser when a user interacts with an ad, more responses to or sharing of image-based ads would likely lead to an increase in revenue. Some on Wall Street have expressed worries about the company’s slowing growth ahead of its initial public offering of stock.

In the third quarter, Twitter had 232 million users who checked the service at least once a month, up just 6.4 percent from the previous quarter and an increase of 39 percent from the previous year. That is far less than the double-digit quarterly growth rates that Facebook posted when it was the same size as Twitter.

Clark Fredricksen, a vice president at the digital research firm eMarketer, said that Twitter’s decision to make its feed more visually attractive makes sense on multiple levels and helps it compete with the image and video-friendly services of competitors like Instagram, Snapchat and Facebook.

"This move may help Twitter more deeply engage users, which is vital for its long-term growth," he said in an email. "At the very least it allows users to perform some of the same actions that helped Twitter’s competitors grow quickly."