Unfortunately this is currently in known issues:
Uploading an image or File button is unclickable
Issue Number: 1862841
Issue Status: In Development
Estimated Resolution Date: 12/10/2019
11/19/19 12:00 PM MST We have found the Add new image or file button in the Email builder is unclickable. This is also in the branding center, template library, Classic email broadcast builder, Legacy email builder in campaign builder, and the webform builder.
You can add your image or Files via the files section. This is found by hovering over the house icon in the upper right corner of the app. Then click on Files. Within this screen you can add company images which will then show up in the builder. So instead of trying to add the image in the builder you can now select the image as it was added via the file section. This goes the same for files.
Product Line: Infusionsoft