Apps have default (?) icon under Linux, OS X; alt+tab is not helpful

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Under Windows the apps have some sort of icon when minimized, but under Linux the icon is some sort of square that I swear I've seen before; I think it is some sort of default. (See attachment)

Under OS X the problem is worse. A minimized window is just a little picture of itself, but the app's icon is always present in the OS X dock while the app is running. The Vespa apps all have the default wx icon which is a rocket ship (see attachment). It's cute but probably not what we want. Furthermore, all of our apps will have the same icon, making them impossible to distinguish.

Also, Alt+Tab under OS X shows a list of the currently running apps. Our apps appear with the rocket ship icon and the name "Python". I can't figure out for the moment how to test Alt+Tab under Windows or Linux because I run them in VirtualBox and the meta keys get trapped by OS X. The same problem might exist under Windows & Ubuntu.

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I figured out that the image is one called Mondrian who is a Dutch painter. Those, Dutch, they're everywhere!