The first two each statements demonstrate that the helper returns a string as expected and that the assets can be found:

{{#each samples as |sample i|}}
{{getLevelImage sample}}
{{/each}}
{{#each samples as |sample i|}}
<img src="{#asset slyk-alert.svg @encoding=dataURI}" style="height:1.75%; margin: 0 auto;">
{{/each}}

Where the helper getLevelImage is:

handlebars.registerHelper("getLevelImage", function(sample)
{
  switch (sample.Results.Level) {
    case 1:
      return 'slyk-normal.svg @encoding=dataURI';
    case 2:
      return 'slyk-warning.svg @encoding=dataURI';
    case 3:
      return 'slyk-alert.svg @encoding=dataURI';
    case 4:
      return 'slyk-critical.svg @encoding=dataURI';
    default:
      return 'slyk-normal.svg @encoding=dataURI';
  }
});

This results in:

But the final line throws the error.

FYI Are you aware that helpers in jsreport can be specified using global function? Calling registerHelper is not required.

function getLevelImage(sample) {
  return 'foo'
}