An Image Tag on a Client is always shown as a URL (weblink). See example in the attached image. The example I have used will never get a Cover Art image via the "Get Movie & TV Info..." function as it has no record on TheTVDB. It is just the News. Besides, if a recording had received Cover Art via the "Get Movie & TV Info..." function an image would show in the Image Tag, along with a reference to where the image is coming from.
The Image Tag usually shows a reference to the Cover Art, not the Thumbnail. If the file does have Cover Art, it will also show the image. The exception seems to be when a Thumbnail is captured from a TV recording or video, where no Cover Art is stored, and only a Thumbnail is referenced. In my case, because I record in the JTV format, the Image Tag reference on the server just points to the JTV file.
CLARIFICATION: I just added a second image for the same TV recording file as the first, but displaying the Image Tag field on the MC Server. No image is displayed as it only has a Thumbnail, no Cover Art, and the Image Tag reference points to the recording JTV file, as below. So in these cases of using a captured image as a Thumbnail for a recording, no image is shown in the Image Tag on the Server, but an image is shown in the Image Tag on the Client, because that image is being served from the MC Server.
There is no way to check a reference to a Thumbnail, which is usually just a smaller, actually one of several smaller, versions of the Cover Art image. All Thumbnails are stored in a single file which is indexed for speed of retrieval.
So, a few things then:
1. If you are connecting to the server via RDP and are still seeing a URL (weblink) that includes "MCWS" as part of the address in the Image Tag, whether an image is shown in there or not, then I think you are still connecting to the server as a Client. You are doing something wrong when you RDP to the server, and actually connecting to a Client.
2. Via the right-click options, Remove Cover Art doesn't remove a Thumbnail if it has been captured from a video frame. That makes sense, as there is no Cover Art, just a Thumbnail. Rebuild Thumbnail will refresh a Thumbnail from a video. I tested that by changing the point at which a Thumbnail is captured from the video. (i.e. I changed the time from 600 to 650 seconds.) You can't directly delete the reference in the Image tag. So how are you removing the incorrect Cover Art?
3. The time position at which a Thumbnail is taken from a TV recording is from the start of the file, and therefore you need to take into account any padding in the recording rule. For example, I have five minutes of padding at the beginning of the example file I used above, and on other recording rules I have ten minutes padding. So I have to pick a time to capture the Thumbnail beyond ten minutes, or some Thumbnails would come from previous programs or commercials before the program. Have you added padding time to your recording rules recently?
4. I didn't think that any EPG data included an image Thumbnail for any of the standard EPG sources. But then I don't use the standard EPG sources. So Yaobing, if the EPG data includes a URL reference to Cover Art, does MC actually use that reference for Cover Art and save the reference into the Image Tag? If so, and that is an external URL, that would slow MC down quite a bit I would think. Although only when the actual Cover Art image is being used, rather than a generated Thumbnail. What EPG source are you using?
So pick a recording example that you haven't fixed yet and post a screen capture like I have that shows the full URL to the Cover Art. Hover your mouse over the URL to make it fully display if necessary before taking the screenshot. I think that would answer most questions.