[Lagopus]

This will give a better result - it will give you filenames like page001, page010, page110 that sorts much better:

echo off
SET /a i=1
SET /a p=1
set /a maxpage=15

:loop
IF %i%==%maxpage% GOTO END

IF %p% LSS 10 (
  rename a%i%.jpg page00%p%.jpg 
) ELSE (
  IF %p% LSS 100   (
	rename a%i%.jpg page0%p%.jpg 
  )  ELSE   ( 
    rename a%i%.jpg page%p%.jpg 
  )
)

SET /a p=%p%+1
IF %p% LSS 10 (
  rename b%i%.jpg page00%p%.jpg 
) ELSE (
  IF %p% LSS 100   (
	rename b%i%.jpg page0%p%.jpg 
  )  ELSE   ( 
    rename b%i%.jpg page%p%.jpg 
  )
)

SET /a i=%i%+1
SET /a p=%p%+1

GOTO LOOP

:end