I've found that "New Screen Recording" functionality in QuickTime Player combined with iMovie editing capabilities is enough for my needs. Maybe you can give it a try? Best of all, it's already installed on your Mac. Unfortunately, I can't compare it with Camtasia, as I haven't used it.

Cool idea! Here's how I'd do it using Final Cut Pro X (or Compressor 4). This won't help if you're looking for something automated. Combination is already been discussed in the comments, so I'll offer more detail on generating stills prior to combining them.

1. How to extract still images from the video using Final Cut Pro X

Here's how to get several stills from a video using Final Cut Pro X. My answer assumes you've imported the video file and added it to a project.

First, add a destination.

Final Cut Pro X doesn't automatically offer you the option of exporting all the frames in a video project. You must "activate" this feature. Open Preferences and select the Destinations panel. Click "Add destination" and choose "Image sequence": Here's what Apple has to say about FCP X destinations.

Second, export or "share" the movie.

Click the "File" menu, select "Share", and click "Export Image Sequence". In the modal export dialogue box, you can adjust file format under the "Settings" tab. I recommend defaulting to TIFF (a lossless format that won't add any compression artifacts).

2. Select the images you want to combine.

Judging from the example above, you're looking for images that are spaced evenly. Perhaps frames 10, 15, 20, 25, 30 will be the ones you need. Pay attention to the quality of each individual shot. If frame 15 is blurry, then try 14 or 16. This could be done using Preview, the Finder, iPhoto, Aperture, or your layer-friendly photo editing software of choice.

3. Combine the images.

As I acknowledged above, I don't have anything to add to the comments already made. Use your layer-competent photo-editing software of choice. Depending on how much the camera moved while filming, you may need to stretch, blend, and rotate each layer.

Considerations

Consideration 1: Interlaced video yields messy stills.

It has been a long time since I've worked with interlaced video, so I don't have much to say. Just know it's an issue with some video cameras, specifically when the resolution is "i" not "p" (e.g., 720i). For example, check out the image on the right:

Consideration 2: Video frames have wayyy lower resolution than still photos.

The bottom line here is that frames extracted from a video will not give you nearly as much detail to work with as still photographs. The resulting "montage"/time lapse will be much lower resolution. (I recognize that the content in question has already been filmed, but for posterity I wanted to point this out.) Unless you're shooting with an incredibly high resolution video camera, like a 4k camera, the number of pixels in one frame of video is much smaller than the number of pixels in a still photograph. 1080p video, the highest resolution on DSLRs, is 1920 x 1080, or 1.98 megapixels. Contrast that to 18 megapixel stills on the same DSLR (e.g. Canon 60D).

Consideration 3: The more the video camera moves, the more difficult it will be to combine your shots.

So use a tripod.