bmp2bic is the classic solution. However, I recently added the option to do essentially what you're describing to my Civ3 editor (link in signature). The advantages over bmp2bic are that it allows any map size (instead of a few predetermined sizes), and that you don't have to do as much work on the image before converting it. You can see an example of what it creates in
this post, and it's now a bit better than that, particularly in that it has support for hills, mountains, and landmark terrain.
It isn't quite copy-paste. You'll need a picture of the map, and then will need to resize it (there is a utility in the editor that will tell you how big the picture needs to be for the Civ3 map size that you want). Then you'll need to save it as either a 16-color or 256-color bitmap file. From there, you load it up in the editor (in a BIQ that doesn't have a custom map yet), choose what colors match up with what terrain, and tell it to import. You'll still need to do some clean-up work, like with any utility, and the amount will depend somewhat on the colors you use in the bitmap file (I recommend using no more than 32 or so - having 256 will take more time and probably won't be much better). But it should get you off to a running start. See
this post for more information, and within the editor, you can also find some help by clicking on the "?" button and choosing the "Import Map From BMP" link (it's slightly out-of-date, but still covers the basic process).
I've only personally used it on pictures of Earth that I got from Wikipedia and NASA. I'd be curious to hear how it does with maps from different games.