This can be also be done with xargs and sed to change the file extension.
ls | grep \.png$ | sed 'p;s/\.png/\.jpg/' | xargs -n2 mv
You can print the original filename along with what you want the filename to be. Then have xargs use those two arguments in the move command. For the one-liner, I also added a grep to filter out anything not a *.png file.