Hello, Rachel,
There is one important thing you need to understand, since you are not a developer. CSS can force a browser to show a big image as smaller, but the image will still be as big as it is. The device will still have downloaded the original image. Our plugin sends a whole new smaller image version to the device, so that the download time is much shorter too! Of course, a developer can combine both of these techniques.
What I am saying is that, no matter what your theme is, our plugin will be able to make your images smaller for mobile devices (as long as a service like Photon is not involved). However, if this will actually show correctly in the device browser is a CSS related issue as well!
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Cheers,
Takis