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.