1.3 had a bug where it treated any image as having the mouseover effect generation unless it was named with _x1.png as part of the name.

1.3.0.1 fixed this so I suspect the issue is the name of your file as if it were right you would have not had the effect on 1.3.  Try naming it "MyButton_x1.png".  Also we have an editor that's downloadable from this forum which will name things correctly for you.

Regarding that theme, on Windows 8 Start8 used the Start menu parts in the OS theme which Microsoft had left.  On Windows 8.1 these parts were removed so the theme does not exist anymore.  But in 1.3 we added the ability for Start8 to have custom themes and we have included two with Start8.  These are not the same as the OS one (copyright concerns may prevent that), but one is fairly similar and third parties can make themes and may well make one eventually that's exactly like Windows 7.

Oh I see the misunderstanding here.

If you name a file with _x1.png it puts in the mouseover and pressed states which are used by the default start button.

In your case if you have custom frames for those you need to name it something other than ending _x1.png.  Ending _x1.png means ignore the other states, always use the first and draw the rectangle under me when required.

1.3 had a bug where it did this for anything not named with _x1.png which was the reverse of how it should have worked which is why 1.3.0.1 fixed it.