Mohaa can load 24-bits and 32-bits .tga files and baseline compressed .jpg files. As a rule of thumb use a .jpg image for a texture that has no transparency.
Just put a texture (width and height in powers of two) in the main/textures folder and it should yet appear in the texture menu.
However, a better idea is to add a shader file in the main/scripts folder. Take a look at the original shaders in pak0.pk3 to see how it's done.
Any questions best be asked

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