How would a defense lawyer argue to exclude photos and video evidence from a trial?
Although this might vary from case to case with the specific circumstances, the basic way to argue for the exclusion of such evidence is to claim that it is not trustworthy.
First of all, it is very easy in today's world to "doctor" photographs and video recordings. A defense attorney could argue that such evidence might not truly show what it purports to show because it might have been altered.
Second, it is quite possible that a photograph (in particular) or even a video recording might not show the totality of the circumstances surrounding what has happened. A photograph shows one moment in time without showing anything about what happened just before or even one moment after. A video recording is also limited in time. It also may not have high quality audio and the picture itself may not be completely clear. If such things are admitted as evidence, juries may tend to give them more weight than they should be given due to their limited nature.
Therefore, a defense lawyer could argue that such evidence is too prone to being altered and is too likely to tell only one part of a given story. Because of this, the lawyer could say, the evidence should not be admissible.
Rather than to simply say that the images/video have been "Photoshopped," a more specific allegation is often required. The defendant may say, "that's not me in the picture," or "that's me, but I was added to the picture." These are examples of copy/paste forgery allegations. These types of forgeries are hard to do well as they leave a trail that can be discovered by image analysts that the court, or the attorneys may employ.
Chain of custody allegations should be specific as well. "The police put that doctored image into my cell phone ..." These allegations can be tested using reliable and repeatable scientific methods as well.
But .. be careful. Most allegations of dirty tricks can be tested scientificly. It may harm your credibility with the trier of fact if you lob acusations baselessly.