Howdy @leona, I don't believe you can playback photos on your TV using a USB cable. For that, you'll need to use the audio/video (yellow and white) cable.
With my Toshiba HD TV I connect using a mini to standard HDMI cable and view recorded videos in full 1920 x 1080. This is provided you set the D3100 to the highest resolution in the menu.
I'm a Medical Electronics Service Engineer with Broadcast TV background, so I'm glad to help on any problems related to digital video or computer matters. Grew up with 35mm film SLRs, which I still own (a 60's vintage Practika and Pentax MZ-50 which is in mint condition). I'm am going to give it to my nephew who thinks that 35mm film is tops and has just been discovered! I'm getting to grips (literally) and exploring the full potential of my D3100 with 18-55mm and 55-200mm lens kit. All the basics like aperture, shutter speed and ISO that I learned on 35mm SLRs has really helped with this. Had two compact digital cameras prior to my DSLR, but was frustrated by their performance and lack of creative ability. I now only take my D3100 when travelling despite the added weight and size. I can definitely say that I'm a DSLR convert and I love my D3100.
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I'm getting to grips (literally) and exploring the full potential of my D3100 with 18-55mm and 55-200mm lens kit.
All the basics like aperture, shutter speed and ISO that I learned on 35mm SLRs has really helped with this. Had two compact digital cameras prior to my DSLR, but was frustrated by their performance and lack of creative ability. I now only take my D3100 when travelling despite the added weight and size.
I can definitely say that I'm a DSLR convert and I love my D3100.