The diffuse map is the image you've already done. It's the base color image. It has a bunch of other names, but diffuse is technically the most correct.
And some shots of the wireframes and lowpoly with no diffuse applied so you can see the sculpt more clearly... Lowpoly - No Diffuse Lowpoly - Wireframes
Tri's are 2,594 with 1024x1024 main body Diffuse texture map, a 512x512 Diffuse texture map for the tank treads and a 256x256 alpha texture map. Let me know what you think.