You can delete images from the Visual Editor on the Edit Posts or Edit Pages admin screens. I recommend editing images when you they are not being used. I have found that beginners need to upload images many times in order to understand how to use the WordPress interface and then run out of disk space. It is also a good idea to keep your site clean and well-organized as it grows.
There are 2 versions of deleting images. You can delete an image from a Post or Page, in which case it will continue to exist in your Media Library. And you can also delete an image permanently from your Media Library.
1. Deleting an image from a Post.
Navigate to a Post or Page, then click on the image you want to delete and click the delete icon. The image will then be removed from this Post but will remain in the WordPress Media Library.
2. Permanently Deleting an Image from the WordPress Media Library.
To view your site’s images click the “Add Media” button.
In the next screen, select the image you would like to delete and then click “Delete Permanently”. Make sure you are viewing your “Media Library” by clicking the “Media Library” tab at the top of the screen. Optional – If you would like to view only the images uploaded to this post choose “Uploaded to this post” from the drop-down menu.
Click “OK” in the alert box, to permanently delete the image from your site.
Deleting Images with the Image Uploader in WordPress 3.4 and below.
To begin navigate to the Post or Page which hosts the image you want to delete>
Click the Upload/Insert icon.
Click the Gallery tab.
Click “Show” on the image you wish to delete.
Once the image and image info is exposed, scroll down and click the “Delete” link.
You are not finished yet. Deleting an image is a two-step process. You need to click “Continue” to confirm the deletion and finally delete the image.
And now you are done and have deleted the image. Repeat the process as needed.