Option 1
Then as you want to use only one line, going with the solution proposed by @choasia with a small change.
<img :src="pointshover ? pointshover : leaderPoints[0].playerimg" :key="pointshover" class="leaderimg">
You won't need the v-if as this image is shown always, and it will have the pointshover src only when it exists and the leaderPoints[0].playerimg when it doesn't.
Option 2
If you go with the two lines code you probably should use:
<img v-if="pointshover" :src="pointshover" :key="pointshover" class="leaderimg">
<img v-else :src="leaderPoints[0].playerimg" :key="pointshover" class="leaderimg">
It's clearer what you want to accomplish if you use an else.
Option 3
You could use a computed property to bind the src.
<img :src="myImageSource" :key="pointshover" class="leaderimg">
myImageSource(){
return this.pointshover ? this.pointshover : this.leaderPoints[0].playerimg;
}