If they value it so much, why send back people? Even accidently sneezing on someone and giving them a cold or a disease from the future itself could have huge disasterous effects on a time time. There is no SMALL action.
Now, pictures are created by a chemical process, the brain creates images and memories on a chemical process as well. Actually, all of our actions are created by chemical reations in certain areas of the brain. Only certain chemicals are affected? If so, then which ones? It would be a good study to see what the human brain or emotions we may lose. If you go to the paradox side of it, the picture never happened, then we would have to use the same analogy for the images or memories in the brain as both are created by chemical processes. Can you confirm which chemical are deleted, removed or affected? I am doing a serious research in Time Travel and since these are supposed to be REAL time travelers, I would like to find out first hand. BTW, I do know special relativity, quantum physics, and biology very well, so a technical explanation with numbers, chemicals, reactions, ect will not be too much of a problem to understand. Thanks for any advice.