First you need to find torrents with less than 10 pictures. For this in the list, put the mouse over the titles and the pictures will display with an indication like "Showing n pictures of x", so fi x is below 10, picture(s) can be added.
Then, go to the description page by clicking on the title and below the pictures is displayed in white and yellow "Add Screenshots and Covers to make some extra SeedPoints" . Just click on "Add Screenshots and Covers".
In the window which opens, click "Browse" and navigate to the picture you want to upload and confirm. Now click the "Upload" button".
The picture will not display immediately, it can take 2 minutes.