Best way to change images in a mod is to open the “image” folder and just replace the artwork with your own.
A Vassal .mod or .vmod file is just a .zip file with a different extension. So, change the .mod or .vmod to .zip and extract the file into and empty folder. You’ll note there’s a new folder called “Images”. You can replace any image in that folder you want. Just make sure the new graphic is exactly the same size (x and y dimensions), the same format (png, jpg, etc.) and filename.
Then, zip everything up, change the .zip to .vmod, and run the module. All the replaced graphics will now appear in place of the old ones.
The best way to do this is to make the new images with exactly the same names as the existing images. Then you can just do as DrNostromo says and copy the images into the zip .vmod file and the new images will show instead of the old images in all cases. This is very simple and quick.
I suspect the problem you have is that you have used new names for your images and your module includes a pre-defined scenario setup. In this case, you need to follow a 2 step process:
Via the VASSAL Editor (or by editing the buildfile if you know what you are doing), update the units to use the new images.
Start-up a game from your pre-defined startup and update it using Tools → Refresh Counters. Then save the new pre-defined startup and use it to replace the existing one in the module.
I wasn’t able to find a Civil War titled module that has a scenario, but the scenario is probably a saved game file with set pieces, which is why you aren’t finding them in at-start stacks in the base module. Any changes you make won’t affect that saved set-up unfortunately… you’d need to re-set the pieces yourself in a new game for that scenario. There might be a workaround to this, along the lines of what was previously mentioned by others, or something else, but I don’t know how to modify or update a saved game file/attachment/extension myself. Hope this helps somewhat anyway.
I’m trying to replace a counter with a modified version. Is there a specific program or method that should be used to unpack a vmod file? I have tried it with winrar and the basic Windows 10 compression tool but when I try to open the freshly zipped version Vassal informs me that the file is invalid. I have tried doing this without making any changes and the problem persists. Meaning, all I do is change the file extension of the module to zip, extract the contents, zip it up again and then change the file extension back to vmod (no files replaced or altered as far as I can tell) but Vassal will not accept the new file. Any ideas?
I have been using 7-Zip which is very quick, ever since Brent put me onto it, and have never had a problem. Before that, I used to just use Windows 10 to treat it as a zip and never had a problem either.
I assume you just want to replace the art? If that’s all you want to do then you could also just go to the counter/piece (in the vassal editor), double click on its image (either in a “Layer” trait or as part of a BasicPiece, depending) and pick a file from your hard drive that you want to substitute - and Vassal will add in the new image for you.
Assuming the modified version of the counter has precisely the same name (take case sensitivity into account), with a good tool like 7zip there is no need to unpack and rezip at all. Simply open the module, then drag and drop the modified image file directly into the zip archive’s images folder.