Moving large pictures files from Windows 7 to new iMac is it better to use WiFi or direct Ethernet connection?

An external drive ... HDD or SSD.

For 50,000 pictures, I'd NEVER attempt wi-fi, instead opting for a wired connection of some sort. But more important than the number of files is their footprint. How much disk space do they occupy?

Get a drive big enough to hold them all and probably 75-100% more for future use. In your case I'd use a high quality mechanical (spinning platter) hard drive.

Using an external drive, flash drive or SD card would be my choice for speed and reliability.

115,000GB. That's a lot of data.