I don't know how you have added it but in general adding max-width: 100% to the CSS should then cause it to automatically shrink to fit the container as the window width reduces.
Your Image does not have the content class, causing the selector “img.content“ not to affect it
There is no use in wrapping it in a <center>-tag if you then declare it to float left
Width: 100% will never take effect, as you declared it width: auto !important before and important declarations can ponly be overriden by other important declarations - without regarding the fact that declaring the same attribute twice within a single selector is “stupid“ anyway as only one can take effect
Remove both width-declarations and replace them with the one from @Brogan, then it will work
PS: Avoid using !important whereever you can. If your attribute doesn't show any effect, you're likely to have either the wrong or a not precise enough selector.