image side [character name] [emotion] = anim.Filmstrip([filename], (x,y), (xF,yF), [time], [total frames], loop=True)

image – Tells Ren’Py that you are about to define an image
side – Tells Ren’py that the image to be define will be a side image
[character name] – Tells Ren’Py which character this side image belongs to
[emotion] – Tells Ren’Py which expression the image will represent

anim.Filmstrip() – Tells Ren’Py that the information within the brackets pertains to a filmstrip animation

[filename] – Where the file is located
(x,y) – The x and y dimensions of each individual frame
(xF, yF) – How the frames are laid out in the file, i.e. the number of frames across and down , reading left to right, top to bottom.
[time] – The time in seconds each frame is displayed for.
[total frames] – How many frames the animation has in total.
loop=True – Tells Ren’Py whether or not to loop the animation

for example:

image side mom happy = anim.Filmstrip("avatars/mom/mom_happy_ani.png", (375,375), (4,8), 0.0625, 32, loop=True)