This is currently not possible. Why do you want this behaviour? Possibly this is something that we can add, if it does make sense for other people as well.

@Michael Vogel to be honest I check my friendica account from a tablet and you will understand that the people around me can more easily see the content... and as I don't have content filters with the other instances, I don't feel very comfortable sometimes with all the images that appear on the timeline and the idea of choosing what and when to see, seems more comfortable to me.
More #privacy

With the new upcoming release we support the "sensitive" flag, which means that we blur images that are marked as sensitive.