So you're just letting the regular query run on that page? No custom query or query_posts? Just the standard loop?
Assuming that's true for the moment, you'll have a line like this..
while( have_posts() ) : the_post();
Update that to..
$i = 0;
while( have_posts() ) : the_post(); $i++;
Then look for the appropriate element you want to change the styling on..
For example, let's assume it looks like this for the moment..
<div class="<?php post_class() ?>">
All you need to do is factor in the counter..
<div class="mainclass back-<?php echo $i; ?>">
Which would then produce two classes for each element..
do all CSS except the background image here
so, stuff like padding, border, width, etc.. (this will apply to all of the posts)
/* the first element background properties */
/* the second element background properties */
/* the third element background properties */
/* the forth element background properties */
If you'd like a more tailored example, post your template code into a pastebin and post the link back, i'll help you implement the code.