usage of file separator

in my code i am iterating thru a list of filenames (like 'image1.jpg'
etc) and appending them to fully qualified directory names

for x in imagefilenames:
imgfile=folder+"/"+x
newimgfilenamelist.append(imgfile)

sothat finally i can get items like c:/mycode/image1.jpg etc from the
newimgfilenamelist

on windows this will work..but how do i make it work on other os?
Being a beginner i couldn't figure out how to solve this using
os.path can anyone help?

sorry if this is a silly qn
dn
.