Updated images with transparent background

[LoznianuAnamaria]

Fixes #1241

Initially, I had the intention to change the background of all visuals that originally had a white background to a transparent one. However, I later noticed that some images within the black theme had become illegible. As shown in the screenshot example, I have now decided to add a white background to all images that currently have a transparent background and are difficult to read.

[codeclimate[bot]]

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