To create a needle just open a new PNG file with the imaging software you like.
Make sure that you have a transparent background color.
No, you don't need to delete settings, background or needles. The uploaded file will overwrite the current one.
The temperature at the left corner is the CPU temperature of the FIS-Control MMI. It shouldn't go above 80°C. Else you should find a cooler location for the FIS-Control box. I will hide the temperature in the coming firmware versions and just display a warning if the temp gets too high. But actually I have not seen any critical temps, even in this hot summer.