Well I'm not a programmer either. I wouldn't have a clue what to change it to in order to display the proper images
I'm only trying to be helpful.
The reason I have learned the little bit of PHP I have is that I have never found a free PHP download that worked perfectly out of the box. The authors intention and your intention may not always be exactly the same.
Also here on the forums please do a search for "Cover Art" or "Album Art" You will find that CDs and downloaded tracks will very rarely have embedded art. What RadioDJ is able to pull from the imported tracks is that much you won't need to search the internet for. You said that you have over 100,000 tracks it may seem that you have a lot. I did the same. The more I sorted through the image files the more I found that the filenames would not be found by web scripts.
In my experience the image filename will be patterned after the track filename not artist-title.jpg
For example if the audio track filename actually contains the artist and tile it will be something like 111_I_Can_t_Deny_The_Light.mp3 so art becomes 111_I_Can_t_Deny_The_Light.jpg. You will need to do some work to correct your image files. Even if the ID3 tag has the Artist and Title of the track the filename itself maybe SID1234.mp3 so the art will be named SID1234.jpg
I grant you that I know the least of the least here on these forums. From what I see The Now Playing from RadioDJ is sending Artist - Title. It will even send Album. It will not send image filenames through the Now Playing from RadioDJ.
If you prefer not to have spaces in your image filenames all you need to do is go down to around line 793
// $artist = str_replace(" ","_",strtolower(($rdj['artist'])));
// $title = str_replace(" ","_",strtolower(($rdj['title'])));
and change to what you need
something like
$artist = str_replace(" ","",strtolower(($rdj['artist'])));
$title = str_replace(" ","",strtolower(($rdj['title'])));
and even add
$title = str_replace(" ","",strtolower(($rdj['album'])));
Again I'm just trying to be helpful