Video answer: Image fit for renpy
Top best answers to the question «Renpy what size image do i use»
By default, Ren'Py will predictively cache up to 8 screens worth of image data. (If your screen is 800x600, then a screen's worth of data is one 800x600 image, two 400x600 images, and so on.) This can be changed with the config.
Video answer: Renpy tutorial #02: displaying and manipulating images