You may use the MHTML format, where the images get embedded as base64 strings. This will generate larger pages though.
The Html5 rendering is meant for our Html5 Viewers and is not intended to generate custom Html5 pages. In this format, each image is indeed saved in a file with a unique name (GUID) and referred to in the corresponding tag. There is no mechanism to specify where the image should come from in the rendered page.
You may log a new feature request for this requirement in our feedback portal. For example, to include an option in the device information settings that allows if the image is referred from and external source (e.g. a file) the 'src' attribute for the <img> tag to be the same source.
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