Why do my screenshots look too dark on other PCs?

There may be calibration tools that you should use. Or you may use a simple program to visually calibrate the brightness of your screen, do something like this:

graphics 512,512,32,2
setbuffer backbuffer()
for i=0 to 255
 color i,i,i
 rect i*2,0,1+i*2,graphicsheight(),1
next
flip
waitkey()

Now adjust the brightness of your monitor until only white is white and only black is black.