-Attention! HL is short for Half-Life, GL is short for GoldSource, tex is short for texture.
Let me take a random image to turn into a Half-Life texture. Remember, if you know how, you can turn any image into a GL tex.
Here we have a Depressionposting meme. I require immediate psychological assistance. But before that, we will resize the image. The rules are simple, the dimensions should not exceed 512, and dimensions all should be divisible by 16. I will help you calculate the best dimensions for your image, using mine as an example.