Problem with ShpEd for terrain tiles

[Gurachn]

I am trying to add a bit more detail to trenches, but am having a few problems.
I can get the bmps out of the .shp fine (I have started with the Z5 version of the summer trench tiles)
I have edited each icon in photoshop, and have no trouble using ShpEd to convert the .bmps to .shp.

After replacing the image in the new .shp and renaming (from BigNew.shp to TER52Z5.SHP) the trouble begins...

Often after I have viewed the icon I want to replace, and open the SP Fix, I find that for some reason the new image seems to be half clipped.
Moving the crosshairs doesn't seem to fix this, in fact the cross hairs often seem to move in the opposite direction to what they seem to be intended to.
This doesn't seem to happen in every case, and I have managed on several trys to replace up to the first 4 icons with no problem, only to find that the 5th is half missing.

Any ideas what I am doing wrong and how to avoid this problem?