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Perhaps this previous answer solves your issue?
Or if your png is large you could try increasing the buffer size as described in the first workaround on Git Push Fails - fatal: The remote end hung up unexpectedly. But as you are using Bitbucket cloud I would be surprised that this is the solution.
We are excited to announce the open beta program for self-hosted runners. Bitbucket Pipelines Runners is available to everyone. Please try it and let us know your feedback. If you have any issue...
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