Captcha Image is not Visible - Customers are not able to Submit in Forms


Hello everyone,

The Captcha Image is not generated in the shop, so users are not able to post comments and fill forms like ‘Sold Out Request’.
So, here is the problem (The code is from one of my forms):

<li class="verify">
							<label class="req">[{oxmultilang ident="PAGE_SHOPVOTE_WRITE_VERIFICATIONCODE"}]</label>
							[{if $oCaptcha->isImageVisible()}]
								<b>Visible Image</b>
								<img src="[{$oCaptcha->getImageUrl()}]" alt=""/>
								<b>Image not Visible</b>
								<span class="verificationCode" id="verifyTextCode">[{$oCaptcha->getText()}]</span>
							<input type="text" data-fieldsize="verify" name="c_mac" value="" class="js-oxValidate js-oxValidate_notEmpty"/>

							<p class="oxValidateError">
								<span class="js-oxError_notEmpty">[{oxmultilang ident="EXCEPTION_INPUT_NOTALLFIELDS"}]</span>

In the code, you can see that I have:
Image Visible - I am getting this message behind the form
[{$oCaptcha->getImageUrl()}] - Here I am getting link like this: and if I try to open the link in browser, file verificationimg.php is downloading automatically.

Any help please?