Yikes. Turns out I broke that in May of 21 when I removed the 4-byte row alignment for images in order to make font storage more efficient. The framebuffer still has 52-byte rows, but a 400-pixel wide will use 50-byte rows, hence the mismatch. Using image:draw(0,0) will always work, but if you want to test how much faster display.loadImage() is you could pad the image out to 416x240 pixels--but then if I decide to change the framebuffer to 50-byte rows it'll break that.
There's probably a good reason I didn't do that? 
Well, at the very least we should note this in the docs.