Why won’t my iPhone 4s sync to my iMac?

Ellie Maybanks Asked: Why won’t my iPhone 4s sync to my iMac?

Recently every time I plug in my iPhone 4s it will not show up on my iTunes, sync photos on iPhoto and at the top where it is charging, the completed charge sign (You know when it's at 100% and the plug comes up) is always showing but instead of battery charge getting higher, it gets lower. Whenever I go to sync photos/videos it always says "Error downloading image. iPhoto cannot import your photos because there was a problem downloading an image." Anyone know how to fix this or what it could be? I really need to sync it as I have videos on there that I need on my iMac for a school project. HELP ME PLEASE!