Maybe I didn't understand correctly, but if you have 2D shape with image mapped onto it as texture then texture coordinates lie in range from 0 to 1 for corresponding vertex coordinates from 0 to image size.
So, tex.x = vertex.x / imageSize.width
, tex.y = vertex.y / imageSize.height
.
PS: may be tex.y = 1.0f - vertex.y / imageSize.height
if image is rendered flipped vertically.