If you right-click on the workspace (not on a view or icon), a context menu pops up where you'll find a Move Workspace Contents item. The same on the Workspace main menu item. Just select the target workspace where you want the contents moved to.
If you just want to move a single image, a better way is perhaps to iconize it and drag the image icon to the corresponding workspace selector (square black icons on the bottom control bar).
The clipboard is the worst way to transfer image data in PixInsight. It is intended only to transfer 8-bit bitmaps to/from other desktop applications.