Hello ttz,
Thank you so much for using our theme AcmeBlog.
You have to write the Custom CSS for this. Please add the below CSS on Custom CSS section. https://doc.acmethemes.com/acmeblog/#WhereandhowtoaddcustomCSS
Please use the below CSS.
.single .single-feat img, .single .single-feat {
width:100%;
}
Note: Child theme is recommended to customize theme.
Besides this we have lots of additional features on our premium version of this theme AcmeBlogPro. Please check our premium product too. http://www.acmethemes.com/themes/acmeblogpro/
If you have further query regarding to our theme, do write us.
Best Regards,
Acme Supports