> Does TP need a specific configuration to be recognized as as admin process?
Not that I know of. Perhaps you could try running it specifically as Admin by right-clicking on the desktop shortcut and using that option.
> Does the drag and drop work for you?
Yes, but I use The Bat as an email client instead of Outlook.
Can you drag and drop from TP to a regular Explorer window?
Are you dragging-and-dropping a regular small JPG using a regular thumbnail (not an orphan or from a gallery)? Could Outlook be in a certain editing mode (eg. plain text vs. HTML) that makes it refuse the file?
If ThumbPlus is overlapping the Outlook window, it may help if (whilst dragging the thumbnail) you first move the cursor to the taskbar and hover over the Outlook button to bring Outlook to the foreground. Then try to drop the thumbnail, and maybe you'll get a better view of what goes wrong.