The most effective method to compress an image in Unity is by using Crunch Compression, as indicated in the following image:

For image Texture Type " Sprite (2D and UI), " you need to make each image resolution like " 256x64, 256x128, 512x256, 1024x512, 2048x1024" like this example image with resolution 256x64 with size 6.4 KB:

If the resolution image is 252x64 will  have 84.0 KB and look like this: