It's a bug!
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"Someone" changed the name of the variable in a java script. In 7.2 the name of this variabele was "baseurl" and in 7.5 it's "script", but "script" is not defined. We changed it back to "baseurl" and MAGIC wink the captcha image appears again!

Refer to image.js:


function newCaptcha(type) {
if (image_pending) return;
image_pending = 1;
get_object('ajax_wait').style.display = "";
//var url = script + "?ubb=captcha&init=1&t=" + type;
var url = baseurl + "?ubb=captcha&init=1&t=" + type;
var ajax = new ubbtAJAX(url, updateCaptcha);
ajax.sendData("GET");
}

function updateCaptcha(responseXML) {
id = responseXML;
obj = get_object('captcha_image');
//obj.src = script + "?ubb=captcha&id=" + id;
obj.src = baseurl + "?ubb=captcha&id=" + id;
image_pending = 0;
get_object('ajax_wait').style.display = "none";
}