To set a sequence of background images for the chess board, choose Settings > Board > Background > Choose sequence folder..., then choose a folder of image files and click OK.
The files in the chosen folder are ordered by the integer between each filename's last underscore "_" and last dot ".". For example:
image_1.jpg
image_2.jpg
image_3.jpg
image_4.jpg
image_5.jpg
image_6.jpg
image_7.jpg
image_8.jpg
image_9.jpg
image_10.jpg
_26.abcd
_42.5qw1er
_99.7u4iop
A simple, human-readable format (such as "image_01.jpg", etc.) is recommended for your convenience.
For animation options, see Choose framerate... and Stop/restart animation.
The images can be any size, and Chess will automatically resize them to fit the board (your original files will not be modified). However, you should try to make sure they are square, as they will be resized to fit a square, so the further they are from square, the more distorted they will appear. You can use almost any filetype you like, including .jpg, .gif, .png, and many other popular formats. See the full list of filetypes at the Pillow reference page.