I'm using this source to get iTunes information for the songs, but it retrieves the cover in .png, and I didn't find where in the script to change it to jpg. Can you help?
The size is normal if you set the resolution so high (best quality search) and this has nothing to do with png or jpg. A detailed picture with a high resolution and a low compression rate is big.
If you do not want a high resolution you have to tell the script to deliver a lower resolution and less quality. To restrict to 1200x1200 change ""100000x100000-999" to "1200x1200".
But I suggest to take the highest possible resolution and later change the picture to your wishes with a software like irfanview.
Yes, there are differences. If you try to download the same image in png, you'll get a much larger file, more then 3Mb most of the times. The same URL with the final jpg will get a jpg file, the same used on iTunes Store.
I don't want the original one in png, I wanted the one iTunes Store uses, 1400x1400 in jpg, which gives me an excellent quality vs file size.
I am using the iTunes source all the time, it is indeed my most used websource, and I never got a png-file, but always jpg instead.
So I don't know how to help.