my startup disk is full - it seems to contain all of my pics and videos ever taken, is this what I need to erase to get more disk space?

You could erase the content, but if you want the photos why not get another hard drive and move the photos and other content to the second drive? That way you can keep the photos, and point iPhoto or whatever you are using to the second drive as the library for the photos. See what they have to offer at OWC, they have good drives and guaranteed compatible with Macs.

You can move your libraries to an external drive (see Ralph's post) - please note: in order for the OS to operate properly, it needs an absolute minimum of 10 - 15 GB of empty space available at all times - more is better. Here are the instructions for moving your libraries (once they're moved and you know everything works, delete them on your internal drive):

http://support.apple.com/kb/PH2302

http://support.apple.com/kb/PH2506

http://support.apple.com/kb/HT1449