Q: Can I replace a figure that is not subject to revision?
My manuscript was judged as minor revision, and it seems to be accepted after revision. In the meantime, there is an experimental image (figure), which was originally rough, but is now clearer after a re-experiment. The image itself has not been required for revision, but can I replace it with a better one?
Firstly, congrats, or at least, pre-congrats on your manuscript possibly being accepted! Also, it is great that you are being so diligent about every bit of your manuscript – you no doubt want it to be perfect, or at least, as close to perfect.
Coming to your query, you should be able to replace the figure. However, you should wait until you receive the formal acceptance mail or acknowledgment. Once you do so, you can write to the editor about this, and in most cases, they should be fine about this, as this is a minor change or enhancement. Technically though, you will be able to make this change now only at the proofing stage. When you receive the proofs and while submitting the reviewed proofs, you can again communicate to the editor that you will be changing / have changed the image.
All the best for all the next steps!