Adjusting image widget, not working

Please help!
I have created an image widget in the header of my site http://www.wrgym.com. I uploaded an image into that widget and everything seemed fine. Now I want to change the image in that widget and replace it with a new one. I have done that using the Appearance>Widget and inserting the new image. At first I could not see the changes on my live site until I emptied the cache on my computer and the browser.

The problem comes when I view my site on another device, computer or tablet. The new image appears on some pages but not all, and in most cases the old image appears on the rest.

Is there anything I can do when I make these types of changes, so that all visitors to my site will see the new images in the widget??

Thanks