Thank you for your message, I would be happy to offer you some suggestions for this. Just to clarify, are you referring to a sizing chart for clothing products? If so, are you having an issue trying to upload an image to your product section specifically? If you are can you clarify what happens when you try to upload that image? Alternatively, you could look into a free third-party app for adding size charts, here are a couple of examples you can check out:
Please feel free to reach back out here if I am misunderstanding your question or you have any additional questions, we are always happy to help out!
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