How to convert an image to DXT? (DDS or KTX file)

With TexturePacker's --convert-texture command line option you can easily convert a single image to a different format:

TexturePacker --convert-texture input.png \
              --pixel-format DXT5 --texture-format ktx

The pixel format specifies the compression algorithm to use, the texture format describes the file container. The following formats are supported for S3TC/DXT/BCn compression:

Pixel format	Bits per pixel	Description
`DXT1`	4	DXT1 compression, 1 bit transparency
`DXT5`	8	DXT5 compression, transparency

Texture Format	Description
`dds`	DirectDraw Surface. Standard format for DirectX applications.
`ktx`	Khronos Texture. Standard container for OpenGL ES textures.
`ktx2`	Khronos Texture (Version 2). Newer format with Basis Universal support.
`zktx`	Khronos Texture (gzip compressed). KTX file compressed for smaller storage.
`pvr3`	PowerVR Texture Format. Native format for iOS and PowerVR GPUs.
`pvr3gz`	PowerVR Texture Format, compressed with gzip. Smaller on disk, decompressed at load time.
`pvr3ccz`	PowerVR Texture Format, compressed with zlib, cocos2d header.

The quantization can be configured with --dxt-mode [DXT_LINEAR|DXT_PERCEPTUAL], default value is DXT_PERCEPTUAL.
For other options (scaling, trimming transparency, ...) have a look at the Image conversion documentation.