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<!-- idea:
Create an SVG that has a raster image and a mask. mask the raster, so that you have a semi-transparent SVG. Include the SVG instead of the "transparent png" in your content.
<svg width="300" height="300" xmlns="">
  <!-- Simple square -->
 <mask id="Mask1">
    <rect fill="transparent" height="100" width="100" y="50" x="50"/>
  <!-- An transparent PNG that is mostly black (so no image shows), but with some transparency -->
  <mask id="Mask2">
      <image xlink:href=""/>
  <!-- A RAT -->
  <mask id="Mask3">
    <path  d="m1.516968,95.256875c0,0 -4.384016,-8.140001 1.75357,-11.679118c6.137586,-3.539007 26.303824,-35.568019 42.086045,-36.098903c0,0 0.876785,-31.851941 12.275081,-26.543321c11.398387,5.30873 6.137586,9.555582 6.137586,9.555582c0,0 19.289453,-27.604979 24.550254,3.185194c0,0 5.26071,7.96304 25.426948,-14.864202c20.166238,-22.827243 89.383297,-28.252739 122.751087,13.802544c21.48137,27.074205 51.292334,67.42007 28.057303,104.049857c0,0 13.450164,80.462153 -98.934839,68.379132c0,0 -43.544047,-9.98387 -65.902202,-8.922102c-22.358246,1.061658 -40.770913,10.086466 -29.810964,-2.123536c10.959858,-12.209892 40.332475,-12.209892 79.788165,-3.185194c39.45569,9.024808 102.584758,7.432156 102.584758,-46.716253c0,0 -9.644726,4.246962 -14.028651,1.061768c0,0 2.191917,11.679008 -10.521511,10.61735c-12.713519,-1.061768 -24.550254,-5.839504 -37.70212,1.061658c0,0 -3.945578,3.185304 -5.260801,0c0,0 -6.575933,0 -4.383925,-3.185194c0,0 -3.726359,-1.858039 0,-5.30862c3.726359,-3.450581 29.810964,-2.919807 28.934179,-3.185194c-0.876785,-0.265387 0.438347,-3.450581 -1.75357,-3.185194c-2.192008,0.265387 -67.074737,5.043233 -84.172182,-1.061768c0,0 -0.876785,7.166769 -4.383925,4.246962c0,0 -32.44141,3.981465 -39.45569,8.493814c-7.014371,4.512459 -4.603235,1.061768 -7.014371,2.123536c-2.411227,1.061658 -5.041582,6.104891 -7.891156,3.185194c0,0 -9.644726,2.919697 -7.014371,-2.123536c0,0 -3.068702,1.327265 -4.383925,-2.123426c0,0 -4.164797,-1.592542 1.75357,-7.432156c5.918367,-5.839504 29.810964,-5.574007 31.564534,-8.493814c1.753661,-2.919807 1.096004,-5.043233 -4.383925,-4.246962c-5.479929,0.796271 -16.659098,-1.592542 -25.427039,-8.493814c-8.767941,-6.901272 -5.930561,-10.33296 -24.550163,-12.740886c-15.051582,-1.946465 -18.120376,-4.600775 -21.919808,-6.370388c-3.799523,-1.769503 -8.767941,-11.679008 -8.767941,-11.679008z"  fill="#fff"/>
  <!-- A Frame: only the white part will show. The black part will be transparent-->
  <mask id="Mask4">
    <rect stroke="#000"  height="300" width="200" y="0" x="0" fill="#ffffff"/>
    <rect stroke="#000"  height="220" width="120" y="40" x="40" fill="#000000"/>
 <image id="image" xlink:href="" height="300" width="200" x="0" y="0" mask="url(#Mask3)"/>


                svg { /* se what part of image is transparent -- will appear grey */
  background-color: #ddd;