If I remember correctly, ffmpeg was quite good at this, and is availble for Linux as well. The first thing that you have to do is to name all your files in a similar matter to this: 01img.png, 02img.png, 03img.png, ..., 20img.png. Now you only have to enter this in the terminal: ffmpeg -qscale 5 -r 20 -b 9600 -i img%02d.png movie.mp4
Where the number after the '-r' flag is your framerate.
Hope I helped.