I dont know about using MCD edit, why do you want to deal with the old MCD format when you can just import graphics into openxcom straight up? Sprite Sheets even.
My advice is just save the images as .GIF rather than .PNG. If it isnt broke dont fix it right?
And custom tailor your palettes to delete colors from them that are off limits, something that is easily recognizable in the image and is unlikely that you would ever use. My suggestion for the color choices are Fuscha (rgb 255 , 0 , 255) for the transparent color and straight red (255, 0, 0) for off limit pixels. Because xcom does not have a good emulation of red in its palettes. By Delete I don't mean actually reduce the color palette choices, I mean replace them with colors that are stupid so you wont use them.
Normally id suggest Teal (0, 255, 255) for the transparency color but it is similar to the blue-seagreen band near the end of the palette and colors might get remapped to it by mistake if you have a bright blue energy blast that you're trying to sprite. It might even get remapped to Teal if you had a bright gray metal sprite with bluish highlights.