Your NetCommunity website is hosted by Blackbaud. When donors review their receipts or when you preview the ereceipt in NetCommunity, images show as a red x.
Attempt to readd the images to the ereceipts. If the error persists and you're hosted by Blackbaud, Click Chat with Support and reference this article. Please be prepared to share the following information with Support:
URL of the donation form in question
Username/Password of the NetCommunity site
In the interim, you may consider uploading the images to an external source, like Google Drive or Imgur, and then drag/drop the image into NetCommunity. This external image will allow the image to successfully display in the Ereceipt.