It's hard to know what the risk is on Unsplash. Probably low. It depends on whether the people uploading the images own the copyright. For the most part, they probably do, but there are no guarantees. On Pixabay, you cannot upload a low resolution file. There's a section under copyright trolling in this article. It explains what happened to one Unsplash user. https://medium.com/illumination/people-are-being-sued-for-using-creative-commons-images-adc5c8ba1491