One of the big negative effects of using stock images is that they will portray you, present you to your clients as generic not the unique person and business that you are, and make you bland in not standing out from the crowd. Think for a second about just how many people will use the same image as the one that you like. They are totally impersonal, not on brand and honestly, they can look fake if not used correctly.
By using stock images, you are weakening your brand. The algorithm in all of the platforms will know that you used a stock image and will push down your post. Google and all the social media algorithms prefer and push up an original authentic photo.