As a side note -- I also considered using a background photo for this, but it shows up in all blocks. If there's a way to have a background photo for just one block that would also solve my problem!

https://www.qualtrics.com/community/discussion/comment/30890#Comment_30890What code are you using for background image?

I'm not using any code, I just uploaded it under survey look & feel. Is there a way to add a background image to a certain block using javascript?

Remove the background image from survey look and feel.
Paste the below code in the first question of the block, where we need to show the background image:

jQuery("#SurveyEngineBody").css({"background":"url('PUT_YOUR_IMG_URL_HERE')",
  "background-size": "cover",
  "background-repeat": "no-repeat",
  " background-position": "center center"});

  jQuery(".Skin #SkinContent").css({
  "background":"none"
});

jQuery(".Skin .SkinInner").css("cssText","background : transparent !important");

Paste the below code in the first question of the very nexy block from where we do not need to show the background image:
jQuery("#SurveyEngineBody").css({"background":"url('')",
  "background-size": "cover",
  "background-repeat": "no-repeat",
  " background-position": "center center"});

  jQuery(".Skin #SkinContent").css({
  "background":"none"
});

jQuery(".Skin .SkinInner").css("cssText","background : transparent !important");