You have just set up your eshop, uploaded your PNG logo and... hey! Why does it have a white background? The reason is the logo has been converted to JPG. And JPG images don't support transparency. How sad.
The solution is very easy.
Go to your back office > preferences > images and check the "image quality" as seen below.
After this, just reupload your logo again and it's done!
Optimal JPEG quality
Another question could - what is the suitable JPEG quality value which is listed below?
The quality should be around 70-75 - The image at low quality (less then 20) and displays very poor color. This would be useful if the image were displayed in a significantly scaled down size - but this isn't this case.
You may find more information about JPEG quality on wikipedia.