How do I move photos stored on external HD back to iMac?

I'm thinking of upgrading my Mid 2011 iMac to a new iMac with 3 TB of storage. My current iMac has 1TB and I have 292 GB left on the iMac. On my external HD, I have 2.31 TB available out of 3 TB. I been able to find a bunch of articles about offloading photos to external HD where the internal HD is not enough space but if I upgrade to the new iMac with 3 TB, I'll have plenty of space and want to move it all back to the iMac. Can someone walk me through how to do this? I can't seem to find anything.

Are your you using Photos on your Mac, and your pictures on the external drive in a Photos Library? If so then just drag and drop the Photos Library file from the external drive into the Pictures Folder on your new Mac.

And if you're referring to folders with photos in them that you are managing yourself (vs having imported them into Photos), then simply connect your external drive to your new iMac, open the external hard drive in Finder and drag those folders containing the photos to wherever you want to keep them on your internal hard drive. That will copy them over. You can delete the originals (on the external drive) if you wish after confirming they all copied over successfully.