First of all, don't use Load Bitmap and then Get Image - just use Load Image - it's a lot easier.
Second, you have the line:
sprite 50,29,(screen height()/2)-6,4
Sprite 50 is probably the one which goes missing - you have used it already running down the centre of the screen so when you use the sprite number again, the first one disappears!
Have you actually considered how many boxes the following line draws?
for m=4 to screen height()-10
Try something like this instead:
set display mode 1280,1024,32
Sync On
sync rate 60
hide mouse
pad_p1_izq=25
pad_p1_sup=280
pad_p2_izq=769
pad_p2_sup=280
m2=5
SH = screen height()
SW = screen width()
color backdrop 0
load bitmap "base.png", 0
get image 2,0,0,7,42
sprite 2,25,280,2
sprite 3,769,280,2
get image 3,6,12,18,24,3
m=4
Repeat
sprite m,(SW/2)-6,m2,3
Inc M: Inc M2,20
Until M2 >= SH-10
Sync
get image 4,6,0,19,13
rem sprite 150,29,(screen height()/2)-6,4
Wait Key
TDK_Man
