The Delta Force game comes with a standard uniforms. I have customised one of them so in the game I see my edited uniform.
I've made a custom uniform with UK flag on the upper arm and "British Army-Jungle Warfare Wing" emblem on the backpack.
Seeing as there is a lot of interest in this I've written up my notes on how it is done. This document describes the steps involved in doing such a project.
The skins are bitmaps (*.bmp). They are contained in *.3DI files. The *.3DI files are contained in the Df.pff file, found in your Delta Force directory.
Note: The changes you make are only viewable to yourself. For someone else to see the changes they have to pack the edited 3DI file into their Df.pff file.
I have a test PLAYER01.3DI downloadable from my website, that has a modified jungle uniform with numbers written on each part of the uniform, and a grid around the upper arm region.
My homepage URL is: http://homepage.ntlworld.com/director/nextdoorskid/index.html