Alnum photos can be inserted in the MP3 ID3v2 tags, and are viewable at least in Windows Media Player, iTunes, ...
It's just a tiny JPEG or GIF image inserted into the ID3v2 tag. Its size (1 or 2 KB) is ridiculous face to the size of any good MP3 (1 or 2 MB)...

There's no way to stop downloading it because it is part of the file content. LimeWire shares files, not fractions of files. LimeWire does not parse files to determine which part is useful and which is not. Instead it parses MP3 to get its embedded meta-data in ID3 (and now ID3v2) tags, that help improving searches by keywords...

So if you stop downloading an MP3 before its end, you may miss some music (an ID3v2 tag can be present at the beginning of the file, and not only at end...)

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