There are two mains reasons that can explain why your logo is not transparent after uploading it to Mojo. 

When you add directly your logo from your iPhone/iPad camera roll, the logo may appears with a white box background format.
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For this reason, the better solution to avoid this problem is to upload your logo from your Cloud files.  
To do so, you have to tap on "All photos": 

Then dropdown arrow and you will see the option "Import from files": 

Sometimes iCloud automatically converts the transparent pngs to jpeg, which might explain why the background is not transparent anymore.
In this case, the best thing to do is importing your logo again from your Cloud files. 

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