Replace image background color with transparent

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.