is a UGUI image with a source image more performant than choosing none?

All my buttons have about 5 images on them, just as decorations. All of them are rectangles, so I was wondering if it's better to use a small white square as a source image instead of choosing none?

 

This is a micro optimization at best. You won't see any meaningful difference on even the slowest of deployable platforms if you see any difference at all, I'd wager.

 

No image is better. You'll need to be bandwidth bound first to be able to see any different in framerate. Even if you're bandwidth bound, you still need to use a rather high resolution image to cause frame drop.

Theoretically, using no images should reduce power consumption IIRC.

 

You mention how many images the buttons have but that's only half of the picture. We need to know the number of buttons you're working with too. Like @Murgilod said this is a micro optimization and any benefits only truly start to show up when working with large quantities.

 

As with every other performance related question, there is no reason to "wonder" whether X or Y performs better. You can simply measure it yourself!