Have a look at these screenies.
They each show what the individual TFormImage parameters do:
Command format
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TFormImage image,a#,b#,c#,d#
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a# stretches the image horizontally

b# shears the image vertically

c# shears the image horizontally

d# stretches the image vertically

The amount of shear/stretching depends on what values are supplied.
As you can see from the screenshots, both positive and negative values can be used.
However, setting a# or d# to zero causes an error.
Some examples:
They each show what the individual TFormImage parameters do:
Command format
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TFormImage image,a#,b#,c#,d#
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a# stretches the image horizontally

b# shears the image vertically

c# shears the image horizontally

d# stretches the image vertically

The amount of shear/stretching depends on what values are supplied.
As you can see from the screenshots, both positive and negative values can be used.
However, setting a# or d# to zero causes an error.
Some examples:
TFormIMage img,2,0,0,1 ; double image width TFormIMage img,1,0,0,-1 ; flip image vertically