I think I know what the problem is, and CSS isn't the solution.
You are 'scaling' the image. In other words, you have an image that is larger than the container in which it resides. The solution is to resize the image in a graphics editor to fit precisely the space where it goes. Presently I see "300px × 300px (scaled to 190px × 190px)" in the image info for your branding logo.
Open the image in some program like Photoshop. Resize it from 300px x 300px to 190px x 190px and rename it. Upload the renamed 190px sized image to replace the larger one. I suspect the shadow will vanish.
Why did this happen? Well, some browsers like IE don't scale images very well. Older versions of IE don't even try to do it. We've answered many topics in this forum for people seeing their post images and post thumbnails bleeding into sidebars or right off the screen, and even breaking pages and posts. All from this scaling issue.
I suspect the Dolphin browser (which I do not know), is having an issue with the size, and creating a shadow, not because of CSS, but because it is having trouble dealing with the scaling.
Let us know if this works, please.