You can upload a standard logo or use a double sized ‘retina’ logo in the Flex theme.
Add a new logo to the theme
1. In WordPress Dashboard, go to Appearance > Customize option.
2. Click to expand the Header Image section.
3. To add a standard logo, click the Add new image button to select and upload a logo image of specified dimensions.
4. For a retina logo, create a logo image that is double the size of the recommended logo dimensions. Then upload the new logo image from the Upload Retina Header section.
If you want to display a full width logo image in the header area (as seen in the Flex blog layout), then use the Retina Header section to upload the logo image.