The frequency you change images creates a sense of speed or urgency.
If you're normally swapping to a new pictures after roughly 5 or 6 lines of dialogue, you might want to increase that to 3 or 4 and later even possibly 1 or 2 during the more peak moments. If it's "normally" 8 to 12, then 6 to 9 then 4-6 then 2 to 3. Just to create a sense of the pace of things increasing.
It's also why sex scenes are frequently animated. Because, again, you're upping the pace.
As far as animations go... no animations is better than bad animations. Animations are a nice to have... and players will always ask for them. But they're optional at best and so if you aren't comfortable with animations... don't worry if you don't have any.