This may happen for one of the following reasons:
Are you using a .jpg or any other format other than .png? In order for your logo to look as nice as possible, you need to make sure that the background of your logo is transparent, not black or white. Transparency can only be achieved with a .png format.
Are you using a .png, but uploading is directly from your iPhone or iPad? According to our information, iOS automatically converts .png files into .jpg, therefore eliminating any transparency in your logo before it can even be uploaded to Phydeo.
To fix this, we recommend using the "Access this page from a desktop Computer" button in your Phydeo page. Clinking on this button will send you an email, which you will use to access a simple web form where you'll be able to upload your logo directly from your computer, therefore retaining your logo's transparency.