@said

You can your own logo image as your Showkase site header by doing the following:
(1) Upload your logo image to the Library ('Library -> Upload').
(2) Go to 'Site -> Customize Theme', click the 'Browse Library' button for the 'Site Header -> Site logo image URL' input and select the image you uploaded in Step #1 above.
You might also want to remove the 'Site -> Site Settings -> Site Header -> Site heading' text.

To make the logo image a link, enter the URL you'd like to use in the 'Site -> Site Settings -> Site Header -> Site header link' input.

The 220px size for the logo is the maximum scaled size in desktop view (in a wide browser window), determined by a percentage of the page width.
Try uploading a larger resolution image for your logo and it should be scaled larger in narrow viewports.

If you want your header logo to be scaled smaller in narrow viewports, try adding something like the following to your '_themes/boma/css/custom.css' file:

@media only screen and (max-width: 768px) {
    .hgroup img {
        max-width: 80%;
    }
}

Hopefully this will help.