dx lines, and such are not elements.
In theory, you just need to draw it before the normal dxes, so in onClientPreRender.

you can use shaders to transform flat image to 3d

@Pirulax Tested and confirmed not working, there is a seperate drawing queue for 3D material elements. However there only is one while there are two for regular 3D lines, one which allows drawing postGUI (someone helped me find it in the source code of MTA itself).

@CrossRoad I don't think it's as flexible as using dxDrawMaterialLine3D? Also I think it makes sense to add this option since it's already available for dxDrawLine3D, a logical addition if you ask me. ^^

https://github.com/multitheftauto/mtasa-blue/commit/bee6c7f245e962541e07a2abfe7ae41ea31f2d73

Please test with https://nightly.mtasa.com/mtasa-1.5.5-full_rc-11998-20180601.exe

Docs: https://wiki.multitheftauto.com/wiki/DxDrawMaterialLine3D

local img = dxCreateTexture("cat.png")
addEventHandler("onClientRender", root,
    function()
        dxDrawMaterialLine3D (0,0,15,0,0,3,img, 7, tocolor(255,255,255,255))
        dxDrawMaterialLine3D (30,0,15,30,0,3,img, 7, tocolor(255,255,255,255), true)
end)

working
https://i.imgur.com/hPeJaFK.jpg

I think you should test the postGui argument too (does it draws the materialLine on top of a 2d dx)

It works perfectly as I wanted.
Thank you!