I am adding optional labels to the matcaps widget. 'Optional' here means that the actual matcaps will look exactly the same, with no text.
The labels would show when using the same widget for preview enums. I expect this template to be more used with the ui-previews: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.75/Addons#UI_Examples

Simply adding text below the image looks great if the image is landscape, but otherwise the label overlaps the image and it's not perceptible.
So, solutions?
What the brushes template does is adding a blue overlay behind the text and over the image. Using the same solution:

Imho it doesn't look very nice. I can try to make nicer overlays with a gradient, but it will look bad if it is only on top of some images or on top of portrait images. With gradient overlays, I would have to make a square crop of the image. This is actually what most galleries use in other programs: big squares and text on top with an overlay.

Third option: have non squared buttons:

In short:

1. copy brushes solution
2. make a better looking overlay, implying the use of cropped square images only
3. have non square buttons with room for icon and text

Again, the use of text in this template would be optional. The actual matcaps will still look exactly the same, squared and with no overlays.

I can apply the chosen solution to both the matcaps and the brushes template so that everything is consistent.
(Note: the brushes icons are 4x UI_size, the matcaps are 5x. should I do something about this?)

Opinions?