Is it an animation? I'll be writing this under the assumption that it is.
1. Video editing software: There's a ton of command line software out there, but I honestly prefer video editing software for GUI. Makes life a lot easier. Openshot is a good free one. So is DaVinci, though I'm not sure if they take image sequences. I personally use Premiere Pro or After Effects, depending on my needs for said animation.
1b. If you're using After Effects/Premiere, then you're going to download Media Encoder. Once you have all those, you'll want to download Fnord. Make sure you install Fnord while AE/PP are closed. Once you have that, you'll open AE/PP.
Select the first image in the sequence (notice all of the images are in order? 0, 1, 2, 3, etc. That's pretty necessary, hence sequence.), and then select the "Image Sequence" box, then click open. This is where Fnord comes into play. Sort of, at least. The rest should be pretty easy to follow via video:
To pull up "Export Settings", just right-click on the black space like I did in the video. After it's exported/rendered, slap it somewhere in your Renpy game directory. What you're going to do after that is define the video as an image, like so:
Code:
image aniname = Movie(play="images/ani/ani1.webm", loop=True, start_image="images/ani/aniname_060.png", image="images/ani/aniname_119.png")
Some notes for the above code. If it's not a looping animation, change loop=True to False. "start_image" should be the first image of the animation and "image=" should be the last. This prevents the black/checkerboard background from appearing. So, now that you have your animation defined as an image, you can play it as if it were a regular render. Like so:
Python:
scene render 3 with dissolve
mc "blah blah blah!"
li "blah blee blah!!"
#animation starts here
scene aniname with dissolve
pause
scene render 4 with dissolve
mc "blahhhh!!!!!!!!"
I think that about covers the process? Could be missing something, but I don't think I am. Openshot should work similarly to AE/PP, sans the need for Fnord. If there's anything that isn't quite clear, I'll try to help if I can.
Edit: Replaced the temporary Streamable link with a YouTube link. More permanent and higher quality.