This is guide is for changing not only Tommy's skin but also any other character in the game. 

You can use this link as a guide to know what file name a certain character corresponds to.

https://thegtaplace.com/forums/topic/16543-names-of-certain-files-in-gta3img/

In this file, I used the 'Cuban Outfit' which you can find in Little Havana

player5 - Cuban Outfit
You will have to change both player5.txd and player5.dff

I also added the cutscence file which is basically the same file which I just renamed as CSplay5.txd and CSplay5.dff. You can add this if you also want to replace the skin in the cutscenes when you wear the outfit.

You can also use this link if you don't memorize the locations of outfits and many other things in the game. You just have to filter it out or direclty search for it on the right side. (e.g. If you type Outfit, it will show you all the outfits name and location on the map.)

https://mapgenie.io/grand-theft-auto-vice-city/maps/vice-city

For this you will have to use IMG TOOL
You can just look for this tool in the tools section in gtainside.com.com

https://www.gtainside.com/en/vicecity/tools/

Step 1
Open IMG Tool

Step 1.1
Again before proceeding, always make sure to make a back up of your file just incase something went wrong. I suggest you copy the whole 'model' folder or if you want you can just create a specific back up file(e.g. gta3.img)

Step 1.2
I will also create a Read Me If You Can pt. 2 for this guide which will tackle how to change back to the original .txd and .dff file. Just remember, this 2 files are inseperable because if there is an error on either .txd or .dff file, you will experience an error in the game itself.

Step 2
Once you open IMG TOOL you have an error that says "Failed to set date for '' ". Just ignore that and click OK.

Step 3
You will see on the top left part 'File' then click it to and click 'Open'

Step 4
Now you have to go to your GTA Vice City folder and look for 'model' folder.

Step 5
Once inside click on 'gta3.img'

Step 6
On the top left part again you will see 'Edit', its just beside 'File'. Then click on it.

Step 7
Then you have to click on 'Find' and type in the file you wanted to replace. (e.g. player5) You can always type in the whole file name like player5.txd or player5.dff but I'm lazy so I just type in player5.

Step 8
Once you found the file you wanted to replace, you will have to delete that file itself since the 'Replace' command in this tools does not work properly. 

Step 9
You can either click on 'Command' and click 'Delete' or do it the lazy way like I do which is just hit the Delete button on my Keyboard itself.

Step 9.1
And when I say 'Delete' button, I am referring the 'Delete' buttone beside 'End' and 'Insert' button not the 'Backspace' or '<--' you above the 'Enter' button but if you have a different type of Keyboard you have to figure it out yourself.

Step 10
Once you already deleted all the file that needs to be replaced. You can then click on 'Command' button again and click on 'Add'.

Step 11
Now you can add the specific file you want but make sure it has the same file name of the ones you deleted before so that there will be no error.

Step 11.1
This time you can add the multiple files at the same time.

Step 12
Once you added everything that needs to be added, click on 'Command' button again but now you will have to click on 'Rebuild Archive'. 

Step 12.1
This is like the 'Save' button for this tool and you just have to wait for it to finish and then you're done.
