Photoshopper (?) comic maker here.
I personally draw each panel separately (usually in a 960x640 canvas), export it as a separate .png and then make a new final file (a long one) of 960x[thousands] pixels, place every panel manually and then export everything as a final .png file.
If the final file is too large, you can always use tinypng.
Now, I've taken the time to write this, be gentle and flair up pl0x :D