Use an art program to make the blank transparent then save as a png, or if you know the rgb value of the blank area you can set the transparency when you load the texture.
Coder, video game industry veteran (since the '80s, ❤'s assembler), arrested - never convicted hacker (in the '90s), dad of five, he/him (if that even matters!). https://deluxepixel.com
i donk know if this "hack" would work on buttons (i think it wouldnt) but in your gameplay if you want to ignore the blank areas you could cut your image in three pieces and load them one next to the other in order to create three bounding boxes.
I use transparent rectangles to do that sort of thing, positioned over the graphic and check for those being touched. In your podium example I would have three rectangles, one for each position, with the rectangles matching as closely as I can the bits that I want to be touched. I usually make the rectangles semi transparent while working on this sort of thing so that I can check their positions.
It's really a variation of what @dreiko65 suggested above where you don't have to chop up your graphics.
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https://deluxepixel.com
http://giderosmobile.com/forum/discussion/218/how-to-get-spritehittestpoint-to-ignore-transparent-areas-of-image/p1
Fragmenter - animated loop machine and IKONOMIKON - the memory game
It's really a variation of what @dreiko65 suggested above where you don't have to chop up your graphics.
http://giderosmobile.com/forum/discussion/3595/non-rectangle-button#Item_7