Merge videos and images using ffmpeg

I'm trying to compile one .webm file that contains this:

1. 10 seconds showing image1.jpg
2. Show a movie (an .mp4 file), which lasts about 20 seconds
3. 10 seconds showing image2.jpg
4. 10 seconds showing image3.jpg

I was unable to find out how/if the concatenate functionality of ffmpeg could do such a thing. Any clues?