Hi John,
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You’re actually referring to the Screen’s BackgroundPicture.
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You’re not placing an Image into a Column into a Screen.
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You need the HeightProperties of the BackgroundPicture set as 20% but the result is that the BackgroundPicture appears extremely small. (But may not be wrong since its size is relative to the physical device screen resolution).
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When you use pixels (How many pixels?) instead of percentage for the HeightProperties, the resulting BackgroundPicture appears correctly sized in the physical device that you’re currently Live Testing with.
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However, the intended / target device for your app is actually another physical device (with a different screen resolution). Thus, you suspect it would not turn out correctly for the intended / target device.
If the above points are correct, I believe you already have the solution in No. 4 i.e. to use absolute sizing in pixels if you want your image to always appear the same size.
Let’s say your current Live Test physical device has a screen resolution of 1242 (W) x 2208 (H) pixels and your intended / target physical device has a screen resolution of 640 (W) x 960 (H) pixels … or vice versa.
So, 20% of 2208 = 442 pixels in Height and 20% of 960 = 192 pixels i.e. it will appear shorter (smaller) in your intended / target physical device … and taller (bigger) vice versa.
However, if you use absolute size in pixels; let’s say your BackgroundPicture is 300 (H) x 300 (W), you’re safe because no matter what the user’s physical device screen resolution is, for e.g., from iPhone 4 (3.5", 640 x 960) to iPad Pro 2018 (12.9", 2048 x 2732), your BackgroundPicture will always display 300 x 300. It will also display in an Apple Watch’s screen as 300 x 300.
Of course in an iPad Pro 12.9", with an absolute sized 300 x 300 pixels image, there would be a lot of surrounding empty space on the screen. So, some people prefer to use relative size in percentage for obvious reasons. See below my BackgroundPicture displayed in iPad Pro 12.9" and iPhone6 4.7" with relative size 60% (H) and 90% (W).
Best is to get hold (borrow or buy [investment as an app development tool]) of the intended / target physical device to Live Test.
