You need smoke or fog in your animated movie? Here are your choices:
1) Use a plug-in or a particle generator... which, on the low-end looks like a plug-in and on the high-end looks great but takes 80 years to render... and if you need the fog to blow around in a certain way it helps to have a degree in physics.
2) Buy a $30 smoke machine, some black paper, and a flashlight:
Adjust contrast and levels, scale, add blur, composite:
These shots are from a dream sequence. Dream sequences in From Inside are animated 12 fps (instead of 24). That's why it looks a little choppy. This is actually helpful for stretching (slowing down) the fog footage. No frame blending necessary.