Be sure to turn up your graphics settings to make sure the shot looks good!
Model viewer shots are deleted on sight (this also includes character select, typically).
Don't include the onscreen text and the selection circle of a NPC.
Don't include any UI in the shot if you can help it.
Use the screenshot cropping tool to focus on the item as much as possible and reduce any unnecessary surrounding, as to better show off the item in question when reduced to the thumbnail that will be present on the item's page.
Reminder