No problem at all! You can add new pictures or delete current ones with a few clicks.
First of all, click the headline of the respective article to see it in full text view. Then click the "EDIT ARTICLE" button.
1. Add new pictures
In order to add a new picture, you need to start a new paragraph. On the left you'll then see a picture icon. Click this icon to upload a picture of your choosing, then click the "Save changes" button in the bar at the bottom of your screen.
Keep in mind that the first picture in the article will always be used for the Thumbnail view.
2. Delete a picture
To delete a picture, you need to mark it with your mouse. It will show a thick blue-ish border if marked. Simply hit backspace (delete) on your keyboard and the picture is gone. Then click the "Save changes" button in the bar at the bottom of your screen.
Please note: Adding or deleting a picture is only possible with a desktop/laptop and not with your mobile device.