Xojo Graphic Overlay - What am I doing Wrong?

I have a large picture with areas masked out in Magenta (&cff00ff)
I have another image (smaller) that is of “grass”
I am trying to replace the magenta areas of the 1st picture with the grass from the second, leaving the NON-magenta parts of the 1st pic as-is

Dim grass As picture
Dim grs As RGBSurface = grass.RGBSurface
Dim prs As RGBSurface = newp.RGBSurface
Dim x As Integer
Dim y As Integer
Dim x1 As Integer
Dim y1 As Integer
For x=0 To newp.Width
  For y=0 To newp.Height
    If prs.Pixel(x,y)=&cff00ff Then prs.pixel(x,y)=grs.pixel(x1,y1)
    If y1=grass.height Then y1=0
  Next y
  If x1=grass.width Then x1=0
Next x

the aspect is all screwed… as a text I took the “grass” and drew a RED “X”, so that the finished image should show that same X, but here is what I get (this is just a small sample cut from the much larger image)


still would like to know if anyone sees my flaw.
in the mean time I just made the grass image bigger than the main image so the coordinates could be 1 to 1, and it worked

I find code MUCH easier to follow with proper naming:

Never used RGBsurface, so:

Maybe use picSurface.Height and .Width?
What’s the bit depth of the picture?
Alpha Channel?

… and maybe -1?

For picX = 0 To newp.Width - 1
  For picY = 0 To newp.Height -1

no, it has something to do whe grassX and grassY… I just made a larger version of grass and took those out, and it worked perfectly

You forgot to reset y1 before starting each nested FOR.

For x=0 To newp.Width
  For y=0 To newp.Height

Your code only works correct if the larger picture is a multiple size of grass? Otherwise the proportions get skewed.