PNG

SVG

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 500 500" width="500" height="500">
  <rect width="500" height="500" fill="#cccccc"></rect>
  <text x="50%" y="50%" dominant-baseline="middle" text-anchor="middle" font-family="monospace" font-size="26px" fill="#333333">500x500</text>
</svg>

SVG (base64)

data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCA1MDAgNTAwIiB3aWR0aD0iNTAwIiBoZWlnaHQ9IjUwMCI+CiAgPHJlY3Qgd2lkdGg9IjUwMCIgaGVpZ2h0PSI1MDAiIGZpbGw9IiNjY2NjY2MiPjwvcmVjdD4KICA8dGV4dCB4PSI1MCUiIHk9IjUwJSIgZG9taW5hbnQtYmFzZWxpbmU9Im1pZGRsZSIgdGV4dC1hbmNob3I9Im1pZGRsZSIgZm9udC1mYW1pbHk9Im1vbm9zcGFjZSIgZm9udC1zaXplPSIyNnB4IiBmaWxsPSIjMzMzMzMzIj41MDB4NTAwPC90ZXh0Pgo8L3N2Zz4=

Dummy image generator