You would need to clarify exactly what you mean by when you use <img src=">, where you are using it, etc., because I don't understand.
But if you replace an image in MC with a new one, but use the same name, MC will continue to show a thumbnail of the old image. Most of the time MC is displaying a thumbnail it creates of an image, and not the image itself.
You need to replace the image with the new one, and then run the "Rebuild Thumbnail" function for the record you are looking at.
For example, if you have replaced the Cover Art image for an audio file with a better image of the same name, then right-click the file and select "Cover Art > Rebuild Thumbnail".
If that isn't what you are looking for, explain what you are trying to do a bit better.