No, setting the default value itself, which is 4 seconds, cannot be changed as far as I know. However when you load an image into the player, a pretty easy way to make sure all your clips are the same duration is to start each one at 0, obviously, as the in point with the “I” key. Then type in the duration you want in the position input, hit enter, which seeks the cursor to that point in time and hit “O” for output. All of your images should be the same duration if you use this method.
If you would like that option to be considered for future versions, just change this topic to suggestion.
Edit: I’m sorry, I was mistaken. When you load an image in the source player and go to the properties panel there should be a option to set the default duration.