I personally believe credibility at first impression has more to do with the overall polish and quality of your design than the existence of a photo.
Sure, adding a photo might be an easy shortcut to fabricate the illusion of quality, but it's a shortcut that also has severe adverse effects on UX in bandwidth constrained environments, which actually has the opposite effect on perceived quality, and thus credibility, than the designer intended. This is why I believe it's worthwhile to explore alternative designs that don't rely on photos, at least once you're past that MVP phase where you just want to put something together as quickly and easily as possible.
RE: You can't relate to a gradient. This is true enough, but there's no reason why you need to make users relate to a background image, when you can have them relate to something in the foreground like some kind of feature showcase of your app/service (with screenshots and such). A busy stock photo in the background is actually detrimental to this kind of design compared to something simple like a gradient. And in my opinion, this kind of design is much more directly relatable for users because you're presenting your value proposition to users explicitly rather than relying on your users being naive enough to fall for some subliminal mind trick.
Stripe's landing page is one good example I can recall of such an approach: https://stripe.com
Stripes target group is developers. Developers, designers etc are special groups, predispositioned in certain way that may make them assess the trustworthyness of an offering differently, i.e. more analytically than most other users.
"Normal" users buy more in the spirit of "no one was ever fired for buying Microsoft": since they lack other means of assessment, or have no time to carefully read the actual content, they fall back on things like nice photos.
And they are a good proxy, as they compare it to advertisements of their local shops or yard sales.
They dont know about stock photos. They just see a professional photo.
And to be clear: Even me as developer would take such things into account. I have incomplete information, so I use proxies. Especially if I am in a hurry.
Sure, but now look at Highrise's visual approach. We know that they have A/B tested in the past, so I am going out on a limb to say that they continue to do so. They have a human element in their hero image on their homepage, and they include an image of a smiling influencer near the top. I'd reaffirm what others have said and test for your audience! I am not affiliated with business of the case study in the link you posted.
https://highrisehq.com/
Sure, adding a photo might be an easy shortcut to fabricate the illusion of quality, but it's a shortcut that also has severe adverse effects on UX in bandwidth constrained environments, which actually has the opposite effect on perceived quality, and thus credibility, than the designer intended. This is why I believe it's worthwhile to explore alternative designs that don't rely on photos, at least once you're past that MVP phase where you just want to put something together as quickly and easily as possible.
RE: You can't relate to a gradient. This is true enough, but there's no reason why you need to make users relate to a background image, when you can have them relate to something in the foreground like some kind of feature showcase of your app/service (with screenshots and such). A busy stock photo in the background is actually detrimental to this kind of design compared to something simple like a gradient. And in my opinion, this kind of design is much more directly relatable for users because you're presenting your value proposition to users explicitly rather than relying on your users being naive enough to fall for some subliminal mind trick.
Stripe's landing page is one good example I can recall of such an approach: https://stripe.com