To upload a photo, the photographer agrees that they have the proper releases (model, trademark, etc.) needed to submit the photo on Unsplash, released under the Unsplash License. You can find more information on Unsplash’sTerms.
You are allowed to use a photo of a public structure as long as you do not infringe on any trademarks. For trademark infringement to occur, there has to be both use of the trademark and a likelihood of confusion that the person using the photo is endorsed by the trademark owner.
For instance, say you wanted to use a photo of the Eiffel Tower. If you were not authorized by the Eiffel Tower to sell tickets and you downloaded the photo and used it as part of an advertisement to sell tickets to the Eiffel Tower, then it’s likely that a consumer would mistaken that you are an endorsed ticket seller by the Eiffel Tower. This would infringe on the Eiffel Tower’s trademark rights. But if you’re a dentist and you decide to print up and frame the photo and hang it on your office wall, it’s highly unlikely that a patient would confuse that the Eiffel Tower endorses you. This would not be an infringement on the Eiffel Tower’s trademark rights.