Is this what you mean?
$n = Read-Host "Enter file name (MainImage.jpg)" Copy-Item -Path .\$n -Destination z:\WEB1 # <=== ASSUMES the current directory (".") in the Path value. Change it to whatever is correct! Copy-Item -Path \path\image2.jpg -Destination "z:\WEB2\$n" Copy-Item -Path \path\image3.jpg -Destination "z:\WEB3\$n" Copy-Item -Path \path\image4.jpg -Destination "z:\WEB4\$n"
The "\path\" part used in the Copy-Item doesn't look correct for a UNC name, and it should have either a "." or a "X:" if it's a file system path (where "X" is, of course, whatever drive letter is appropriate)