Themes for Xoops come in all shapes, sizes, colors, and styles. It would be impossible to have a one size fits all logo for all themes.

Each theme has its own image folder that contain images that are defined in the theme.html for that specific theme.

Inside this image folder is an image usually named logo.*

You will have to make a new image the same size as the original, move your new image to the image folder replacing the original. Your image should have the same name as the original, unless you want to edit theme.html to point to your new image's name.

For each theme that you want your users to be able to switch to you will have to repeat the above steps.