Thank you for recording a video demonstrating the issue on your side.
It is really a strange issue which I am not able to reproduce in a test Sitefinity 6.2 project. It seems that the link to the image is not properly resolved. This may happen if you have moved the image to another location or if you have changed the url of the image. In this cases when you select the images in the Sitefinity's built-in ContentBlock widget which is using the HtmlField the link is resolved automatically. However, when you use the LongText field as a custom field in a module the link to the image will not be automatically resolved.
Can you please take a look at the following blog post
where this issue is described in more details and try to use the suggested approach there for resolving the url by using the LinkParser class:
//get the html from HtmlField and convert it in formt usable for ResolveLinks method
var fixedHtml = LinkParser.UnresolveLinks(Name);
//resolve the links if there has been a change in the url
NameLiteral.Text = LinkParser.ResolveLinks(fixedHtml, DynamicLinksParser.GetContentUrl,
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