If I understand you correctly, this .bat file should do the trick:
Code:
echo off
SET /a i=1
SET /a p=1
SET /a maxpage=10
:loop
IF %i%==%maxpage% GOTO END
rename a%i%.jpg page%p%.jpg
SET /a p=%p%+1
rename b%i%.jpg page%p%.jpg
SET /a i=%i%+1
SET /a p=%p%+1
GOTO LOOP
:end
Edit the value for "maxpage" so the script stops when you're out of pages. This will rename your images like this:
rename a1.jpg page1.jpg
rename b1.jpg page2.jpg
rename a2.jpg page3.jpg
rename b2.jpg page4.jpg
rename a3.jpg page5.jpg
rename b3.jpg page6.jpg
... and so on.
(No guarantes. Run it on a test folder first.)