No you need to edit the corona texture itself using graphics software such as Paint.NET. Essentially what I do is select Open in Explorer, - then open the corona texture in Paint.NET then make a copy of it. I then reduce the size of the copied image by 20-25% and paste it over the original making sure the smaller corona image is placed at the centre. I then tidy up any stray original image bits that weren't covered over and save it to the asset folder. Close the folder and select Submit. Once the changes to the asset has been accepted take a look at it in the preview window to see if it's what you want. If not adjust as necessary and repeat.
Others might have a more efficient way of doing this, but I like working in small steps since I'm less likely to mess up.
