Question

This seems so easy, I’m embarrassed. Every time I trigger this it just stays in the “On” state. it doesn’t ever pass the conditional to turn off.

-(void) lightbulbSwapImages
{
    NSImage *bulbOn = [NSImage imageNamed: @"sidebar_lightbulb_on.png"];
    NSImage *bulbOff = [NSImage imageNamed: @"sidebar_lightbulb_off.png"];
    if (lightbulb.image = bulbOff)
    [self.lightbulb setImage: bulbOn];
    else
    [self.lightbulb setImage: bulbOff];
}

Answer 1

Change = with ==.

- (void)lightbulbSwapImages
{
    NSImage *bulbOn = [NSImage imageNamed: @"sidebar_lightbulb_on.png"];
    NSImage *bulbOff = [NSImage imageNamed: @"sidebar_lightbulb_off.png"];
    if (self.lightbulb.image == bulbOff)
       [self.lightbulb setImage: bulbOn];
    else
       [self.lightbulb setImage: bulbOff];
}

Reason behind your issue:
What you are doing is, trying to assign bulbOff to lightbulb.image and the result you are checking as a condition which will always return true if lightbulb.image is not nil. Due to this it will never execute your else part.

It is equivalent to,

lightbulb.image = bulbOff;

if (lightbulb.image) {

On a side note, if you are using @property for lightbulb, make it consistent by using the same everywhere including the if condition. In my answer, I have changed from lightbulb.image to self.lightbulb.image.