Well, GIFs are limited to 256 colors no matter what, minus one color for transparency setting, leaving you with 255 colors at the picture palette (all pictures within an animated GIF need to have the same palette or else it'll screw the colors while converting). IrfanView tries to calculate an avarage color of all colors there and makes a palette for the GIF to make every color above 255 a color that matches one of the existing ones best... kinda hard to explain...
In short: You won't have problems with sprites like your avatar image is, but you most likely will with pictures (like JPEG conversions or something), because they usually have more than 255 colors.