Change size of blog post picture and text box width - make smaller size picture with narrow width paragraph

Requirement is for Downsizing Image on Blog Detail Page but as we are working with image and its CSS values to customise the image on the page but the change on Blog Detail page effects the Blog Landing page too.Second issue when we change values of image from 100% to 75% the hover vale does not change it remains 100%.

Please let us know how we can effectively manipulate the image from where and how. Stepswise would be appreciated.