PNG-files in icon box are blurry

I wanted to change the icons in my icon box by uploading my own png files. However they are displayed quite blurry compared to the original files. The size of the pictures is 125 x 125 px and I already uploaded them with that size (not blurry). As soon as they're in my media they look blurry. 

Here's the link: www.grandega.de (it's the second part right under the main picture. 

Is there any way to fix this? 

Thanks in advance.