I could point out that DelphiX source is right there If you know how, you can experiment things like removing that dithering. If it works please tell all the details so it ends up in next version of UnDelphiX. It's just DirectX when it comes to core of it

ps. DXDraws.pas is the unit that renders all and handles the textures.

Edit: And 1 more thing, try what happens when using different image sizes. For example 3D cards are most optimal for square textures with size power of 2 (mostly 64x64, 128x128, 256x256).