Yes it's normal. If the image doesn't fade out over time though it's not normal. Our screens are LCDs and this is common for all LCD displays, TVs included.
Plasma and OLED displays have it worse because the phosphor wear on these can be permemant. I used to play a certain game hours and hours each day on my old Galaxy Nexus which had an AMOLED display and I eventually had the menus permanently burned into the display.
Since our G2s use a LCD display though any image persistence we may encounter should fade away once the screen is off for a while. If it doesn't then no that's not normal. Most image retention issues should fade away in a few minutes, but rarely it may last up to a few hours to a few days.
Unless you keep your display on a static image though hours and hours on end, this shouldn't be an issue. If any retention doesn't fade away after having the display off for a while or if it commonly happens without having to try to make it happen then you might consider a warranty replacement.
There's a ton of information regarding this if you search on "screen burn in image retention oled lcd plasma" in Google.
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