The D3100 is capable of taking the pictures you desire.
The unidentified lens attached to it might be also.
The embedded EXIF file tells me the Gator picture was shot at:
Focal length = 70mm (105mm 35mm equivalency)
Shutter Speed = 1/50 Second
(Too slow for handheld with 70mm lens)
f/Stop = f/4.2
ISO = 3200
(Too fast - High speed not needed for still shots - Adds electronic noise to photo)
White Balance = Auto
(What color is background, white or beige?)
Use a tripod and remote shutter release (or camera's self timer) to minimize camera shake.
Use diffused lighting, either a Light box or outdoors on an overcast day.
Match camera's White Balance to color temperature of light source.
Manually select lowest ISO setting (film speed) for best detail definition and lowest noise.
Polarizing filter will reduce reflections and increase color saturation. Try with and without, and pick what you like best.
Numerically higher f/Stop (Smaller aperture) will increase depth of field while lower numbered f/Stops will make focusing more critical. Middle of the road would start at about the f/4.2 range.
The nice thing about taking pictures of inanimate objects is that you can keep trying different things until you get what you want, or give up trying.
Try different lighting, different camera settings, different angles and different distances. Most of all, take tons of pictures.
Hope this helps.
Good luck.