How to wrap image with div in typo3?

According to the documentation for the CONTENT object:

Since in the above example .renderObj is not set explicitly, TYPO3 will automatically set 1.renderObj < tt_content, so that renderObj will reference the TypoScript configuration of tt_content. The according TypoScript configuration will be copied to renderObj.

Since you declare SCREENSHOTS.renderObj.dataWrap = <div class="screen">|</div> you properly wont have any configuration automatically copied to your renderObj.

If I understand your goal correct then you wish to remove all wraps around 'image only' content element and wrap it all in one div tag <div class="screen">|</div>. The following is untested but should work if your provided first code blocked worked for all your 'image only' content elements.

# Create a new renderMethod named 'noWraps' which can be used across the whole system
tt_content.image.20.rendering.noWraps {
  imageRowStdWrap.dataWrap = |
  noRowsStdWrap.wrap = |
  oneImageStdWrap.dataWrap = |
  imgTagStdWrap.wrap = |
  editIconsStdWrap.wrap = |
  caption.wrap = |
}

SCREENSHOTS < styles.content.get
SCREENSHOTS {
  select.where = colPos = 9
  renderObj < tt_content
  renderObj {
    # Set renderMethod to 'noWraps' only for this section
    image.20.renderMethod = noWraps

    # Change the default wrapping around the content element but only if it's a 'image only' contant element
    stdWrap.innerWrap >
    stdWrap.innerWrap.cObject = CASE
    stdWrap.innerWrap.cObject {
      key.field = CType
      default < tt_content.stdWrap.innerWrap.cObject
      image = TEXT
      image.value = <div class="screen">|</div>
    }
  }
}

If the above code dose not work then just write a comment and then I will take a look into it.