Guillaume's answer is good. I just wanted to add: don't do this:
image= [pos1];
All that does it to destroy the built-in function called image(), and nothing useful whatsoever in your code. It can be eliminated without any problem. Never use "image" as the name of a variable.
By the way, another possible method to find the lowest row is
lowestRow1 = find(sum(BW1, 2)>0, 1, 'last'); % Bottom dot in image #1
lowestRow2 = find(sum(BW2, 2)>0, 1, 'last'); % Bottom dot in image #2
output = [lowestRow1, lowestRow2]; % The output array you said you wanted.
Basically you're summing the image horizontally (across columns, which gives a mean vertical profile) and finding the last (lowest) row that has something in it. However I'm not sure why you want 343 and 423 when the lowest rows are 342 and 423 - why add 1 to the first image???
