)
In brief, just open it in the source application, select a web friendly ColorSpace (like sRGB), or remove the ColorSpace completely, and re-save it.
How you do that depends almost entirely on what you used to create it. I mostly use PhotoShop, but it's changed again and I can't remember if it's easier to do it in the Save dialogue, or if you have to go into the preferences to change it.
Whichever app you use though, searching for "ColorSpace AppName" should find the steps though.