[chacam]

Can an iMac (desktop) pick up a wifi signal or does it need an ethernet cable?

[memason]

The iMac can do WiFi, so no cables required.

[mitchbr47]

Originally Posted by chacam

Can an iMac (desktop) pick up a wifi signal or does it need an ethernet cable?

Yes. All imacs, even those original ones built in the 90s have wifi built in. Today's models use the faster speed of IEEE 802.11n. The letter n is important to look for as it is the fastest speed.
You can also use an ethernet cable if location to the modem isn't an issue and the cable isn't unsightly to you.