The issue you are seeing might be because of the X,Y coordinates that you are using in the image control.
Here, Parent.X/Parent.Y refers to X/Y coordinate of gallery with respect to the whole Screen, if you put it as is in the image control, it binds the coordinates to the Gallery outline and not the whole screen.
ex. Gallery Coordinates: X:10,Y:235 with respect to whole screen and now if the height of your gallery is 150, and if the image Y coordinate is set to 235, it will definitely not be displayed as it does not exist in the current gallery bound limits, if you will increase the length of gallery, lets say to 300, then it will be visible. Its all about the relative coordinates on the control.
Hope this Helps!
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Actually, the Parent.X and Parent.Y formula in the Image control within Gallery would return the X /Y coordinate value of Gallery with respect to the current Screen. But the X/Y property of the Image control is respect to the Gallery control rather than the screen.
In addition, the .TemplateX proeprty and .TemplateY property is not supported within Gallery control. If you would like this feature to be added in PowerApps, please submit an idea to PowerApps Ideas Forum: