to modify the video you have to edit it using a program. I'm not sure if you use windows or mac but they both have great simple editing software which would allow you to have writings, pictures, and even edits on the voice.
you can also subscribe to few great video software companies that will allow you to take your video and edit it to perfection but that might be too soon.
If you have iphone i know you can use adobe edit on your mobile to edit the videos. i'm not a big fan cuz the small screen and big fingers don't go together. lol