How to add images to the categories of my WordPress

What theme are you using? A link to your site so someone can see what you are talking about would be helpful too.

Where exactly do you want to add the images?
DO you want to assign an image to a category or you want to assign an image to a post?
If it is a post you have to “Add a featured image”.
Which theme are you using?

When i go to the categories page i can′t see the pictures. The theme that i am using is oxygen and WordPress version is 3.6.

Here is the link to my categories page. The content can be seen but images don′t show. https://demotosonline.com/category/accesorios/