Coordinates scaled to image dimensions
ImageScaled[{x, y}]
ImageScaled[{dx, dy}, {x0, y0}]
Graphics[Circle[{0, 0}, ImageScaled[{.5, .25}]], Frame -> True]
→ -Graphics-Graphics[{Disk[ImageScaled[{.2, .4}], 1]}, Frame -> True, PlotRange -> {{0, 10}, {0, 10}}]
→ -Graphics-Graphics[Disk[{0, 0}, ImageScaled[{.5, .25}]], Frame -> True]
→ -Graphics-