Your photos are what other collectors see when they look up a mug, often the only picture of that design anywhere. None of this needs a real camera: a phone by a window and a minute of care is the whole trick.
Get close enough that the mug takes up most of the picture. On a mug page your photo is shown small, and a mug lost in the middle of a table is unreadable at that size.
Mug fills the frame
Lost in the room
A blank wall, a clean table, a sheet of paper: anything that doesn't compete. Clutter behind the mug makes the design harder to read and dates the photo.
Plain, even backdrop
Busy background
Glaze is a mirror. A flash bounces straight back as a white blob over the artwork, and overhead spotlights leave hard shadows. Face a window instead, on an overcast day if you can.
Even, soft light
Flash blowout and hard shadow
Landscape, not portrait. Nothing is ever cropped: the site shows the whole photo, in a wide 4:3 slot, and fills whatever is left over with a blurred copy of the picture itself. So a portrait shot loses nothing except size: blurred bars down both sides, empty ceiling and empty table above and below, and a smaller mug for exactly the same effort. A mug is a wide subject; give it a wide frame.
Landscape, as big as the slot allows
Portrait, bars either side, smaller mug
The city or country artwork is what identifies the mug, so it should face you square-on, with the handle off to one side.
Artwork square-on, handle aside
Handle hiding the design
Stand a little higher than the mug: enough that a sliver of the inside shows, so it reads as a real object rather than a flat cut-out. Much higher than that and the opening turns into a wide oval while the artwork foreshortens into a band.
A hint of the inside
Too high, the design flattens
The order matters: your first photo of a mug automatically becomes your cover for it, the one shown on your profile, and on the mug page itself if nobody has photographed that mug yet. So lead with the shot that shows it best.