See the blend
video filter:
ffmpeg -loop 1 -i input0.png -loop 1 -i input1.png -filter_complex "[1:v][0:v]blend=all_expr='A*(if(gte(T,3),1,T/3))+B*(1-(if(gte(T,3),1,T/3)))'" -t 4 frames_%04d.png
This example will make a 3 second cross-fade of input1.png
over input0.png
.
To crossfade/dip-to-black multiple images see Create video with 5 images with fade-in/out effect in ffmpeg
.