To edit or remove sponsors first of all go to your home page, while logged in as a site administrator.
You will see the black tool bar, usually to the right of your screen.
Select the first button - "edit content" and you will see the list of content items on the home page. Select and click on the one called "Sponsors". This may vary in position depending on the layout of your page.
You will see a content block showing your current logos.
To delete a sponsor, just click on it and press delete
To add a sponsor just click to the right of the last sponsor in that category, and then use the image manager in the toolbar to upload a new image. The image manager has a symbol as shown below. The size of the logo is not particularly important, but try to have the smallest dimension greater than 250px and the largest less than 400px where possible.
DON'T FORGET TO CLICK SAVE when finished