You might consider getting an external enclosure for the hard drive in your PC. You could then connect the drive directly. I don't know if the Windows Migration Assistant supports that, but you could follow the manual instructions.
If you decide to move your pictures manually to iPhoto, here is a nice tip:
Inside the iPhoto Library (and iTunes, too) is a folder named Auto Import. If you drag photo files into that folder, they will be automatically imported into your iPhoto Library. To get into the Library folder, ctrl-click on the iPhoto Library and choose Show Package Contents.
Now, you could just choose Add to Library and point it to the External Hard Drive with you PC hard drive in it.
If you want to move things into iPhoto organized into Albums (folders) from an organized folder structure, you can drag a folder of photos onto the Albums label in the iPhoto Library list on the left sidebar and it will create an Album, named with the name of the folder, holding all of the photos that were inside that folder. You can drag a folder into the Events list and it will create an Event with that folder name. I don't know what it does with subfolders, though, as there isn't such a structure in the iPhoto Library.