What's probably happening here is that the size of the Captivate stage is very different from the PPT sizing, and that's causing some loss of resolution when it scales.
If you have Captivate 6, try using the new High Fidelity import setting.
If on an older version of Cp, see if you can modify the setup of the PPT file so that it matches the Captivate slide. Perhaps that will help. However, (and I know this is NOT what you're going to want to hear) for the ultimate solution that would keep your images at highest resolution, you'd probabl need to build the presentation in Captivate from the start using the same assets.
I've got the stage and PowerPoint the same size, that's not the issue.
I'm aware of the High Fidelity setting, but it's not showing up on mine. Is there a setting that needs to activate that?
I'm coming to terms with your "rebuild it from scratch" scenario. It's like the five stages of grief...
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