I have a Sony DSC H5 and it has pretty much the same settings - mine is on series which is I believe as you think it is and the photos keep going up in sequence number
I also have an older Sony DSC S85 - I let its battery die for a few months and when I recharged it it had reset back to the beginning of the sequence
In any case this is not an Apple or iPhoto issue - iPhoto is using the names that you Sony is assigning - and it is either a error in the camera, a battery issue or a camera setting
Until your figure your camera issue out
You can resolve the issue for the photos in your iPhoto library by exporting your photos as JPEGs (file menu ==> export) rather than dragging the out and using the output options to set new file names before you drag them to your USB drive
LN