So this is how i got it work for me. (sorry in advance for the long comment lol)

in Open IV, go to mods\x64v.rpf\models\cdimages\streamedpeds_players.rpf\player_one

*don't forget to hit edit mode*

in the search bar in the top right , type "uppr_diff_015 ", you're gonna be replacing these. hold off for now, we'll come back to these in a sec.

for the PNG files , you'll have to resize the image you want to 1024 x 1024 so they end up fitting properly. i saved them with the same name in my downloads folder to make them easier to find later on. (*if you have windows* double click on the image, once it opens up click the 3 dots on the top left, click on width, type 1024, the height should automatically change to the same number, then save.)

once you save them, go back to open iv and double click on one of the files ( uppr_diff_015_a or b or c etc), it'll open up the OpenIv texture editor. On the upper right click replace, find your new resized pic from wherever you saved it (downloads folder for me as an example), click on it and hit open. then save on the bottom right. rinse and repeat for the ones you wanna replace.

*If you wanted to use the DDS Image files*

i ended up having to download paint.net to convert them to PNG files. you can download it here ( https://paint.net/download.html , i clicked on the dot.pdn one and installed like normal) *those DDS Image files should have automatically changed to PAINT.NET files*

once paint.net is open, go to file , open and find the new paint.net image file you want then click open.
once opened, click file again, click save as and change the "save as type" to PNG File and save. click okay on the save configuration. (you should now see the new png file in the shirt folder)
to install these , you would do the same steps as above to resize, replace , etc.

*to equip using the rampage trainer, go to player-wardrobe-torso . Change the type to 15 and then change the texture to which ever new shirt you want*

I'm not sure if there's any other ways to do it, but this way worked for me with a little help from ChatGPT so i figured id pay it forward and help someone out.

happy trails !