In your Property Inspector on the left side of the page there is a feature that allows you to set an image as a page background. If an image is set as a background, it is essentially locked and you will not be able to move it. You can however click that same button to detach the image from the background which will resolve that issue.
Additionally, there is a new feature that allows you to lock layers, which would also prevent you from moving assets on the page. Clicking on the lock icon to unlock the layer would solve that issue.
If neither of the above work, try selecting that layer in the layers panel. If you are able to move the image with your arrow keys, it could be there is another layer on top of the image layer that is blocking you from accessing that layer. You can then move the image above that layer and it should solve the issue.
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