You can replace images in one place(A) on one page, or replace image in all the places where this image is found(B).

A: Replace one image on page

• Select image or rectangle with background
• Right click on image or rectangle
• In opened context menu click “Replace Image” for images, or “Replace Background” for backgrounds
• Select new image and click OK

B: Replace one image the entire site

• Open Assets panel (Window/Assets or Ctrl+Shift+D)
• In Assets panel select the image you want to replace
• Right click on it. In context menu select “Relink...”
• Select new image and click OK

Note: Some images and backgrounds in theme are PSD files, so you can edit it in Adobe Photoshop.

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• Muse
• It's me