You have to take into account the pixels in your image. The upper left corner will have values of 0.0 (xpos 0 ypos 0); if you are using 1920x1080 images, the lower right corner will have a value of 1920.1080 (xpos 1920 ypos 1080).
You can also use xalign or yalign, where 0.0 (xalign 0 yalign 0) is the upper left corner, and 1.1 (xalign 1 yalign 1) is the lower right corner; then xalign 0.5 yalign 0.5 will be the center.
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Also keep in mind that those positions are relative to where you are using them (full screen, grid, frame, etc).