Hello,
It's possible to convert all kind of files to dds format that is readable by ESO using ImagickLibrary for example in PHP:
Code:
$im = new Imagick();
$im->readImage( "sourceFileName.jpg");
$im->setImageFormat('dds');
$im->setCompression(imagick::COMPRESSION_DXT5);
$im->setOption('dds:cluster-fit','false');
$im->setOption('dds:compression','dxt5');
$im->setOption('dds:weight-by-alpha','false');
$name = 'outfilename.dds';
file_put_contents ($name, $im);