"There was an error processing your order. Please contact us or try again later." error on magento

I am developing a website with magento and for somereason I am getting this error and email when I try to check out.

Error :

"There was an error processing your order. Please contact us or try again later."

Email

Payment transaction failed.

Reason
Image is not a PNG

Checkout Type
onepage

Customer:
xxx

Items
Gift Certificate x 2 AUD 10
test product x 2 AUD 25
Gift Certificate x 1 AUD 10
test product x 2 AUD 25
test product x 1 AUD 25

Total:
AUD 155

Billing Address:
abhinab kayastha
ghj
gjhghj
g
jhgjhg, 7897
Togo
T: 234

Shipping Address:
abhinab kayastha
ghj
gjhghj
g
jhgjhg, 7897
Togo
T: 234

Shipping Method:
Free Shipping

Payment Method:
Check / Money order

Date & Time:
May 1, 2012 2:47:40 PM 

Thanks Ab

This sounds like there's an issue while creating a PDF file (probably the order PDF).

The error is thrown because at some point of the PDF creation Magento tries to include a .png file, which does not have the usual PNG signature in its first three bytes (like "real" .png files should).

See lib/Zend/Pdf/Resource/Image/Png.php.

Check your var/log/exception.log or set

Mage::setIsDeveloperMode(true);
ini_set('display_errors', 1);

in your index.php to get an exception trace, which should help you to pin down the file causing the error.