Also make sure the captcha image tag passes all of the WA_Globals variables:
<img src="WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php?noisecolor=<?php echo $WAGLOBAL_Captcha_Noise; ?>&bgcolor=<?php echo $WAGLOBAL_Captcha_BG; ?>&textcolor=<?php echo $WAGLOBAL_Captcha_Text; ?>&transparent=<?php echo $WAGLOBAL_Captcha_BG_transparent; ?>&characters=<?php echo $WAGLOBAL_Captcha_Characters; ?>&width=<?php echo $WAGLOBAL_Captcha_Width; ?>&height=<?php echo $WAGLOBAL_Captcha_Height; ?>&font=<?php echo $WAGLOBAL_Captcha_Font; ?>&gridfreq=0" alt="security code" />