The search on the front of your Magento installation is displayed without going through the Magento back-end. This means the extension has to generate the url for images at indexing time, not at the time of searching. When the extension generates this image, Magento will provide it with a cached and resized version of the image to display. One of the possible reasons that images are missing, is that this cache could not be generated automatically by the Magento installation.
First, please make sure the latest version of the extension is installed. If the version of your extension is lower than 1.5.x, please update.
There are two main reasons why a problem with images would occur.
Images appear and then disappear This problem usually means that the image cache has been dropped. This is often triggered manually from the back-end of Magento (System > Cache Management) or through the command line. When the image cache is cleared, indexed image links become invalid because the file will be nonexistent. A full reindex should fix this problem.
Images are not showing at all This problem usually means one of two things. Either there is an issue with the directory permissions, or there’s a memory issue.
First, make sure the media/ directory has the correct permissions. The permissions should be set to 770/660.
If this doesn’t work, enable logging.
With logging enabled, run a full reindex and open the log file after to see what’s going wrong. If nothing useful comes from the log files, please check the Magento and Apache/Nginx log files as well to check if the problem lies there.