A very simple testing function I made that takes any image from a variable, shrinks it—in a way that preserves most dithers—and overlays it on the screen in one of 9 slots (on a 3x3 grid).
Pick any image, any abitrary slot that doesn't block things you need to see, and specify some accompanying text which gets printed to console along with the slot # for reference.
Call it like this:
imgtest(8, myImage, "white block")
The code:
function imgtest(slot, img, printText)
--Overlaid 1/3-size image for testing
--Positioned in slots 1 to 9 on 3x3 grid
--Must do gfx.sprite.update() in playdate.update()
playdate.graphics.setBackgroundColor(playdate.graphics.kColorClear)
local y = math.floor((slot + 2)/3) - 1
local x = slot - y * 3 - 1
local s = gfx.sprite.new(img)
s:setCenter(0,0)
s:moveTo(x*400/3, y*240/3)
s:setScale(.33333333)
s:add()
print("IMGTEST "..slot..":", printText)
end