Go to https://pixlr.com/editor/ and select "Open image from computer". Select image from computer
Select Want tool and click on the solid background color
Go to Edit and select "Invert selection"
Copy the image to clipboard
Create new image and select you want to create layout from clipboard resolution. Make sure to check "Transparent"
and finally paste the content from clipboard
Save the project
Make sure to choose "PNG (Transparent, full quality)"
P.S. If you have multiple background areas while select background using "wand tool" then hold ctrl key and click one by one on them all.