As per https://jira.atlassian.com/browse/CONF-37623, images cannot be attached to global templates. They also cannot be dragged in.
However, images can be pasted in, which will work fine until you attempt to save. When you attempt to save after pasting an image in, the save will fail for no reason and with no feedback to the user.
Confluence should either warn images aren't able to be pasted in, adjust the image so it references the page / URL it came from or strip the image out.