The real question here is:
The key is to use image style effects in the proper order.
First of all you need to install the Image Cache Actions module.
Lets say that we need all logos in a carrousel to be 100px by 100px.
So you have to imagine that you will have a square figure of 100px by 100px and inside of it you will place the logo by scaling it to fit proportionally.
So hands on it.
And that’s it.
Now you have all logos scaled and centered inside a 100px by 100px transparent square.
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