FFmpeg encod set of time stamped images as Iframes
Your understanding of what an encoder does is incorrect. It doesn't make up images to fill in the time between I frames. It does somewhat the opposite: it takes a full series of complete images and decomposes them so that only the I frames and the differences between them remain.

What you're asking for is, in animation terms, called tweening. The linked article will point you to some software that can do automated inbetweening, but it's usually not useful for real-life images over the durations you seem to have. For smaller intervals (a frame or two) the process is called interpolation.