Document Preview from HR Agent Workspace, Canvas Error

Hello experts,

After creating a HTML document template, I was asked if the HR Agent could add their signature image to the document before passing the completed document to the employee. Instead of having a static image in the document template, it was suggested that they add their signatures to the letters per case creation.

I've found that when I "preview" the document in the HR Agent Workspace, I can edit it; however, there is no icon available to insert an image. I decided to try and copy/paste an image of a signature into the document and when I do, I get an error: "Failed to upload image: Request failed with status code 404". There is also an error message: "Canvas Error: An error has occurred. Please refresh or contact support."
Funny part is, after those messages, the image is there and resizable; and when I choose to save it, it is successfully saved. And it just works.
After that point, anytime I choose to preview the document again, so long as the image is there, I see the same error messages; But all appears to be working. 

Is this a known issue? Anyone have any workarounds to get rid of the error messages, or possibly enabling the insert image icon into the editor?