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Once you have a Base64 encoded string, use it for the src attribute's Value. Be cautious when embedding images directly in a form. A GlobalForm can not exceed 15 megabytes in size. In general, even very complex forms are no where near that size. If large images are embedded directly into a form however, there is a chance you can reach or exceed the maximum size for that form.
For your Base64 equivalent encoding make sure you have added the image type information to the beginning of it. Like this for a PNG file. data:image/png;base64,
Truncated Example of Value data:image/png;base64,iVBORw0KGgoAAAAN..........................
