Save as to your desktop. They are 135*60, you may need to adjust to the size you want. When you select the gauge option in sensor panel manager, select custom. You can then assign each image from 0-15
If you use rslcd format panels its easier than you think. There's sites out there that will let you resize, crop or reverse gif images for free. Ive used ezgif quite a lot for this.
1480 * 320 based on the themely modern clean panel. Please feel free to adjust to your own requirement, panel is for personal use only not for resale.
themely 1480 320.sensorpanel