Ah, I see.
You need another way to make screenshots. Recall that in LemEdit, instead of just scrolling, you can explicitly type in where you want to scroll the level view's x position to. It is the box labeled "Sp:".
Since the screen width is 320 pixels, this means all you have to do is:
1) enter 0 as the value for "Sp:" (or, scroll to extreme left), make screen capture
2) enter 320 as the value for "Sp:" make screen capture, join it immediate right of previous screenshot
3) repeat step #2 with the values 640 and 960.
4) The next number would've been 1280, but because the total visible width of a level is only 1584, LemEdit would convert this last screenshot to start from 1264. Keep that in mind as you append this last screenshot to the full screenshot. It shouldn't be difficult if you preset the full screenshot bitmap's width to 1584, that way you just need to right-justify the 1264 screenshot.
5) Do the usual beautifying like cropping etc.