In DEVONthink for macOS, version 4.1.1, I can place an HTML file in a group, and in that file, reference an image that lives in a sub-group. The HTML file will render perfectly well, showing the image.
This was the same in DEVONthink for macOS version 3. I’m not sure how it’s done, because individual files in DEVONthink for macOS are stored in various Finder folders according to their types, as defined by their extensions, and the simple path defined in the HTML file is not the same as the actual Finder path between the HTML file and the image. But I guess DEVONthink implemented some trick to maintain the intended path. It works in macOS anyway.
It also used to work in DEVONthink To Go version 3.x, which was pleasing, but it doesn’t in the new DEVONthink To Go version 4.0.2. In DTTG version 4, the image appears as a generic image icon.
(Oddly, the preview icon for the file in DTTG (the blurred preview before it’s downloaded) does show the image, and it’s also discernible in the tiny preview of the HTML page in list view; but the image doesn’t render in the HTML file itself.
Why is this disparity apparent between the platforms’ implementations? Is it intentional?