How to put a picture on someones head in a video

You should probably use TrackX from CoreMelt for this. Don't export the video, it'll just make it more difficult. Apply the plugin to each clip separately to track each shot. You could do this manually keyframing the emoji position, but it sounds like it would be a huge amount of work.

As a small addition to what Tom said: depending on how much "perfection" you require, this can sometimes be done "by hand".

Add the circle image as a connected clip, and add keyframes to match the movement, more or less. Of course, this is easy is there is not much movement, and harder if the head moves a lot. You essentially do the tracking "by hand". On plus side, it is free :-)

It is not so hard if it just needs to be in the ballpark...

First, you can use Preview to get the virus and letters and remove the background. It is slightly easier to do with Pixelmator or another still image editor, but Preview, again, is free and already on your mac. Use the magic wand to remove most of the background and then use the selection tool to handle the rest. Two minutes is enough.

Then try it on one of the clips. Place it above the clip, adjust to match the start of the video and add a keyframe. Then move the playhead a little further, and when it starts to be out of place, adjust (a new keyframe is added automatically, you just need to have placed the first keyframe). Repeat.