This should be an option that can be altered from within the theme editor. Depending on the theme you are using, the position for this setting can change. If you are using the Dawn theme, or a new 2.0 theme, then you should be able to open the theme editor, select the section from the left hand side bar and then adjust the slider that appears in the section settings on the right hand side (see image below).
It is usually referred to as an image overlay. If you aren’t able to find that adjustment slider, let me know what theme you are using and I will try to have a look myself.