Version 1.07 of this plugin is deceptive, poorly documented, and buggy. It is deceptive because a Vertical Playlist shortcode is produced, but does not work unless you buy the paid version, which you don't know unless you saw the note about "Paid Features" in the description, you noticed the small "Paid Vertical HTML5 MP3 Player with Playlist" in the caption on the screenshots page, or you visited the support forum. If a feature is not operational, there should not be a UI for it, or it should be clearly marked with something like "Only works in paid version". At least a meaningful error message should be produced. In addition, nonremovable and nonmodifiable twitter, Facebook, and developer site links are displayed above the playlist, but not shown in the screenshots. This is deceptively advertised under "Free Features" as "Social Links Facebook / Twitter".
The plugin is poorly documented because of the confusion over the items just mentioned, but usage in general is unclear, and the image size isn't specified. Trial and error is required.
The plugin is buggy because looping is inconsistent, playlist items disappear when clicking to advance to the next track, and the settings page has refresh issues that appear when using the unneeded scrollbars. On the settings page scrollbars appear on the right and bottom of the form field area, even though the browser window is more than large enough for all form fields. This makes the settings page hard to use. These scrollbars are not needed, and it appears they are there to keep the "Donate" button visible.
In summary, this plugin is not quite ready to be used and so cannot be recommended until the issues above have been fixed. Some of these issues have been brought up in the support forum, but the developer responds with phrases like "quality is not free" and "try paid version", so it appears that there is no future for this plugin unless someone else wants to take the code and fix it.