Black background on some images

Hello,

I am trying to create a gallery for different client logos and wanted there to be 5 columns of logos per row. All the thumbnails and images are the same size and they were not being configured in the proper grid. After a day they did align themselves to be in the proper format but now on three of the images there is a black background that disappears when the image is moused over. Is there any way to fix this?

Thanks

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