I was working through this issue today and found this post because I was also looking for a solution. 
Base64 is too much text for the GPT to work with when you have a regular size image. The URL that is returned by the DALL-E Image Generation capability is not valid outside of the chat window.
I don’t know if this will help your use case, but I did get my GPT to pass a DALL-E generated image URL to an action. I did this by unchecking the DALL-E Image Generation Capability and wrapping a call to the api.openai.com image generation API in an Action.
Since it’s the API and not the Chat window, it will cost me for each image generated, but I believe it’s only about $0.02 per image. Not ideal but it let me get on with my day.