If you try to drag and drop a layer below an expanded image service layer, the dropped layer does not get added and the table of contents (TOC) switches to the Source tab.
Image service layers are implemented as a group of sub-layers. This composite layer type does not implement add or remove items; thus, ArcMap does not recognize the area below them in the TOC as a drop zone for adding layers to the map.
Collapse the image service node by clicking the minus sign to the left of the layer name in the TOC, then drag and drop the layer below it.
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