I recently received an iMac as a gift. I have always used PC and cannot figure out how to get my 11000+ pictures from my PC to my iMac. I tried using an external hard drive but it doesn't seem to connect. Any help would be greatly appreciated :)

What format is the external hard drive using?

You need to format the external hard drive as FAT32 so the Mac can see it, but single files are limited to 4 GB.

Copy as much as you can over to the new computer. Then, repeat until all photos are on the Mac.

Do you have a WiFi network or any network? You can connect to the IMac through the network and transfer your pics that way.

Or just burn a DVD of all your pics and put the DVD into your iMac.

Well that's not good about the external drive.
Maybe burn them to DVDs?
Or use something like dropbox? Dropbox gives you 5 gigs of free space - upload them from PC into your dropbox, then go on the mac and download them.
https://www.dropbox.com/referrals/NTgxMzYwMjk?src=global9

I find using an iMac very pleasurable by operating it with a large axe. Attaching the external drive to the head of the axe will give it extra velocity on the down-stroke and maybe, just maybe, some of the pictures might be transfered to the iMac.

Or maybe not.

If I remember right they both have USB and Ethernet ports. Just hook up a USB to USB cable and one or the other should recognize a computer hard drive at the other end of the cable, and essentially you can drag and drop files from one HD to another. There's also Ethernet where you can create a local network and make a 'Shared' folder which you can then copy to the other computer.

Alternatively, you could get a flash drive, but with that many photos it may take a while. There's also uploading/downloading via cloud or mediafire/dropbox, but that seems pretty useless if the other comp is just a few feet away.