If you don't have a graphics program, you can download paint.net. It is easy to use and is free. I just put text on a picture, as an example.
It is helpful to have the cover fit the confines of whatever device you are wanting to display it on. Don't for instance enclose both the picture and some text in a paragraph together, if it will exceed the screen size vertically. It will make it spill over onto the following page.
<p> the image</p>
<p>the caption</p>
works better, though the caption can be separated from the picture when displayed by the reader. But better than it spilling over.
You do need to add the picture to the epub, then insert the picture.
And if it is a cover, right click on the file name in the book browser and add sematics, cover page.
This two step of add and insert is necessary so that the image can be reused, since it exists itself in Sigil.