Ok, i found the solution.
All the information provided above were correct and helped me to solve it completely. Thank you!
The central issue was that i do my changes in the wrong file! I think that the template "Afterburner2" doesn't use neither the core files (under _sitename\layouts\joomla\content\intro_image.php) from joomla nor the files for override (under _sitename\templates_templateName\html\layouts\joomla\content).
When you want to change the intro_image part and you use the "Afterburner2"-Template you have to edit the File under _sitename\plugins\system\gantry\overrides\3.3\2.5\com_content\category\blog_item.php. On Line 123-132 you can find the part for the intro_image.
<?php if (isset($images->image_intro) and !empty($images->image_intro)) : ?>
<?php $imgfloat = (empty($images->float_intro)) ? $params->get('float_intro') : $images->float_intro; ?>
<div class="img-intro-<?php echo htmlspecialchars($imgfloat); ?>">
<img
<?php if ($images->image_intro_caption):
echo 'class="caption"'.' title="' .htmlspecialchars($images->image_intro_caption) .'"';
endif; ?>
src="<?php echo htmlspecialchars($images->image_intro); ?>" alt="<?php echo htmlspecialchars($images->image_intro_alt); ?>"/>
</div>
A working solution looks like that:
<?php if ($params->get('access-view')) :
$link = JRoute::_(ContentHelperRoute::getArticleRoute($this->item->slug, $this->item->catid));
else :
$menu = JFactory::getApplication()->getMenu();
$active = $menu->getActive();
$itemId = $active->id;
$link1 = JRoute::_('index.php?option=com_users&view=login&Itemid=' . $itemId);
$returnURL = JRoute::_(ContentHelperRoute::getArticleRoute($this->item->slug, $this->item->catid));
$link = new JUri($link1);
$link->setVar('return', base64_encode($returnURL));
endif;
?>
<?php if (isset($images->image_intro) and !empty($images->image_intro)) : ?>
<?php $imgfloat = (empty($images->float_intro)) ? $params->get('float_intro') : $images->float_intro; ?>
<div class="img-intro-<?php echo htmlspecialchars($imgfloat); ?>">
<a href="<?php echo $link; ?>">
<img
<?php if ($images->image_intro_caption):
echo 'class="caption"' . ' title="' . htmlspecialchars($images->image_intro_caption) . '"';
endif; ?>
src="<?php echo htmlspecialchars($images->image_intro); ?>" alt="<?php echo htmlspecialchars($images->image_intro_alt); ?>" />
</a>
</div>
<?php endif; ?>
It is not exactly like the solution here Similar Solution
You have to conform the code a little bit.
Thanks for each Tip i get!
Additionally i want to say, that i think i edited the core files which the Template afterburner2 uses...it is not a override i think. I will look for it, but if somebody know a better way to do, for example a override, i would be glad of another solution.
After a little more research and antoher hints from FFrewin (again "Thank you" at this point :)) i think a found another solution without edit the core-file!
I copied the file "blog_item.php" from the Folder _sitename\plugins\system\gantry\overrides\3.3\2.5\com_content\category (where the core files stored) to the following location _sitename\templates\_templateName\html\joomla\3.3\com_content\category. When the file is stored there and contains the modifications i described before it also works for me.
The Folder _sitename\templates\_templateName\html\joomla does exist but in there only the folders "3.0", "3.1", "3.2" does exist. Because i use the the Joomla-Version "3.3.6" at the moment, i have to create the folder "3.3" manually. In the folder "3.3" i created another folder "com_content" and in there a folder "category", so that i ended at the path i told before. In the folder "category" i copied the file blog_item.php from _sitename\plugins\system\gantry\overrides\3.3\2.5\com_content\category.
I think that is a better solution, because you must not touch the core files.