I'm wanting to paint a mirror frame that [I believe] is covered by laminate (It's your basic run-of-the-mill frame ... holds a mirror, but could easily hold a picture/photo).

Do I prime, first?
Do I need to sand, then prime ... ?

TIA!

Yup, can be painted. Clean it up well... scrub the HECK outta it. Rough it up with sandpaper. Prime... I like Kilz. Paint and seal.

Since you're talking about a picture frame, paint should hold up VERY well, cuz not getting and real "wear".

I painted 2 bathroom countertops in a faux marble a few years back. They have held up very well. It's just me most of the time, so don't know how it would last with a house full of kids who may leave puddles or wet towels on the surface.

I painted a laminate and wood table for my daughter. I didn't prime or sand. I used Krylon Fusion for plastic spray paint. It has worked for me for 3 years, just one scratch despite 2 moves and lots of lovin from a now almost 5 year old.

No need to sand. Just clean it really well and then use BONDING primer. I love the SW product, but I understand that Zinsser also makes an excellent bonding primer for about half the price.

I have painted lots of laminate this way and it holds up to tons of abuse. No chipping, peeling or bubbling.