When you need test images, instead of using services like lorempixel Show archive.org snapshot or placehold.it Show archive.org snapshot you may generate test images yourself.

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Here we build a simple SVG image and wrap it into a data: URI. All browsers support SVG, and you can easily adjust it yourself.
Simply set it as an image's src attribute.

JavaScript

Simple solution in modern JavaScript, e.g. for use in the client's browser:

function svgUri(text) {
  let svg = `
    <svg width="320" height="240" xmlns="http://www.w3.org/2000/svg">
      <rect x="0" y="0" width="320" height="240" style="fill: #ddd; stroke: #666; stroke-width: 4;" />
      <text x="50%" y="50%" font-size="24" text-anchor="middle" alignment-baseline="middle" fill="#666">
        ${text}
      </text>
    </svg>
  `
  return 'data:image/svg+xml;charset=utf8,' + window.escape(svg)
}

See the attached JSFiddle for an example.

Ruby

If you like to do it on the server, here is some Ruby for you:

def svg_uri(text)
  svg = <<~SVG
    <svg width="480" height="270" xmlns="http://www.w3.org/2000/svg">
      <rect x="0" y="0" width="480" height="270" style="fill: #ddd; stroke: #666; stroke-width: 4;" />
      <text x="50%" y="50%" font-size="24" text-anchor="middle" alignment-baseline="middle" fill="#666">
        #{text}
      </text>
    </svg>
  SVG

  "data:image/svg+xml;charset=utf8,#{ERB::Util.url_encode(svg)}"
end