When shopping direct-to-consumer fashion brands for new jeans, dresses or even shoes, it can be challenging for consumers to figure out what size will fit best, and what color or pattern to buy. Without having the option to physically try on the item and see it in real life, consumers are often left to make a best guess based on size guides or compare their measurements with those of the model wearing the clothes, when those are given.

Some online retailers are finding a way around all the guess work, making it that much easier for shoppers to find the perfect fit and thereby converting more sales. Brands from Rent the Runway to Lulu’s are seeing big benefits across the board using customer photo reviews: pictures the customer posts of themselves wearing the item, usually with the size purchased and the customer’s measurements posted alongside the photo.

Rent the Runway started using photo reviews as early as 2012, after finding that shoppers were sending photos of themselves in a dress they rented to the customer service agent who helped them select the style for an event. “From there, we built photo reviews on the site, and it’s been extremely successful. It’s consistently quoted as one of our most favorite customer features, and we have almost 1.5 million customer reviews live on our site now,” said Anushka Salinas, chief revenue officer at Rent the Runway.

Salinas said photo reviews provide some shopping inspiration, allowing customers to see how other women have styled the product. They also help shoppers see what a style looks like on someone with a similar body type. On Rent the Runway’s site, customers leave their height, weight, bust size, the size that they normally wear and the size they’re wearing in the photo.

For customers of Lulu’s, an online women’s fashion brand, the benefits have proven pretty similar. “In general, the big question when online shopping is just: Will this fit me?” said Colleen Winter, CEO of co-founder of Lulu’s. “If you’ve never shopped at Lulu’s and have no idea of the sizing, you don’t know if it’s going to fit. Plus, having customer reviews with photos has really allowed us to showcase diversity of sizing and body shapes.”

Having a diverse range of body types modeling clothes wins brands like Lulu’s and Rent the Runway major points with shoppers, who have grown tired of seeing airbrushed and photoshopped near-perfect models. “We all know that not everyone has a model body, and sometimes it’s hard to compare yourself to someone who seems absolutely perfect,” said Winter.

Photo reviews also have big benefits for the brands themselves. For Rent the Runway, the company can take the nearly 17 million data points it has collected from photo reviews to do a few things: One, it can understand which products and brands are popular with shoppers and worth stocking up on, and which ones aren’t. The reviews also help Rent the Runway work closely with designer partners to share any fit or quality issues.

“If an item is not of perfect quality or is not holding up to a dry-cleaning process, or if the fit is imperfect in certain areas, we go back and show our designer partners and help them understand any issues that our customers might be experiencing,” said Salinas.

“Generally, when a brand is working with a retailer, they’re not getting this type of feedback” she said.

With the feedback, brands can improve their designs and products, which Rent the Runway can then buy again, and sell or rent more of that improved product in the future. For Lulu’s, constructive criticism typically helps the brand improve the quality and fit of styles, Winter said.

Lulu’s has found that when items have photo reviews attached to them, they tend to sell more, although the brand declined to quantify the difference in sales. “We’ve done so many improvements overall to the site, so it’s really hard for us to pinpoint what is driving the sales,” said Winter. “We’ve increased our site speed, changed up our product detail — we’ve done a bunch of things. I can tell you that when products have more reviews with fan photos, the product has a higher conversion rate.”

The trend isn’t just for e-commerce-focused brands, either. American Eagle and sister brand Aerie both allow customers to share their own photos and thoughts on purchased items, or items they’ve tried on in-store. The company did not respond to a request for comment, and it’s unclear when exactly they launched this tool.

In the future, both Lulu’s and Rent the Runway said they’ll continue to improve the photo review features both online and on mobile. Rent the Runway recently rolled out the reviews on mobile, while Lulu’s is in the process of developing and launching its own app, which will include photo reviews. The two said they’ll be working on integrating more of their own stylists’ photos into the mix in the future.

“When there aren’t reviews, people ask for them. They ask why we don’t have photo reviews on every product,” said Winter. “It’s because they’re user-generated, and we have to wait for them to roll in. So we’re going to start having stylists that work for us, and have varied body types, try on and take pictures and post them.”