I follow these considerations when creating STDA*. files:
- The image resource "ins2" should have a certain WxH that results in a integer value when divised by 8, for example:
on Jaguar JR9 "ins2" has 48 pixels width and 34 pixels height:
48x34=1632 -> 1632/8=204
the result 204 has no decimal value, so it should be ok.
I also use this rule on "whl1-3".
- The image resources "ins1", "ins3", and their associated alpha masks "inm1" and "inm3", have the same width and height dimentions has "ins2". In this case they all have to use 0,0 (x,y) starting coordinates since they are all referenced by "ins2" starting coordinates (wiki).
If "ins1" and "ins3" ("inm1", "inm3" respectively) have smaller images than "ins2", they can be placed on other starting coodinates, but they are all relative to "ins2" (check STDAFGTO.3SH example), and the rendered image cannot overlap "ins2" image area.
There are other considerations that have not been fully clarified. It turns out that when I edit these image resources to make a perfect rendering in the context of CGA or EGA game setup, there is some misalignment rendering "ins*" and "whl*" that I think it has relation with the width x height pixel proportions. (check for example Caterham Super 7 JPE in EGA)

- The image resource "ins2" should have a certain WxH that results in a integer value when divised by 8, for example:
on Jaguar JR9 "ins2" has 48 pixels width and 34 pixels height:
48x34=1632 -> 1632/8=204
the result 204 has no decimal value, so it should be ok.
I also use this rule on "whl1-3".
- The image resources "ins1", "ins3", and their associated alpha masks "inm1" and "inm3", have the same width and height dimentions has "ins2". In this case they all have to use 0,0 (x,y) starting coordinates since they are all referenced by "ins2" starting coordinates (wiki).
If "ins1" and "ins3" ("inm1", "inm3" respectively) have smaller images than "ins2", they can be placed on other starting coodinates, but they are all relative to "ins2" (check STDAFGTO.3SH example), and the rendered image cannot overlap "ins2" image area.
There are other considerations that have not been fully clarified. It turns out that when I edit these image resources to make a perfect rendering in the context of CGA or EGA game setup, there is some misalignment rendering "ins*" and "whl*" that I think it has relation with the width x height pixel proportions. (check for example Caterham Super 7 JPE in EGA)