What you're ultimately trying to create is an 8-bit color bmp file, using the specific palette of colors that bmptoctp2 looks for. It needs to be 2x the linear size that ctp2 uses. For example, for a "Gigantic" map (which ctp2 says is 70x70, although it's really 140x70), you would create a 140x140 bitmap. The conversion program then skips half the appropriate lines to generate the map. It has an option that is supposed to let you use a 140x70 map without skipping any pixels, but I haven't been able to make it work properly - it looks like the horizontal size is one pixel off, so you get this weird diagonal banding effect. This could probably be overcome with some experimentation.

So, the fun part: How do you create this bmp file? What I do is start with a graphic file of the map I want to make, of any arbitrary (but considerably larger than the final) size. I keep the original on one layer, then start coloring in the terrain on layers above that. If you keep different features on different layers, you only have to worry about "staying in the lines" once. For example, I usually have (in order from top to bottom):

Oceans
Rivers
Terrain Features (mountains, forest, etc)
Terrain Base
Original

I'll usually start by creating the oceans, by tracing around the coastlines with the "Shallow Water" color (tip: have the terrain types bitmap that comes with bmptoctp2 open in a second window, and use the color picker to get the right colors). You'll want to use the "Pen" rather than the "Brush", because you don't want any blending between colors. Also, since you're using a file larger than the final map will be, you don't need to worry much about little fiddly bits (they'll disappear when you resize anyway).

Then, continue with the various terrain layers. Since you've got the different "things" on different layers, you can turn the display of layers on and off depending on what you're working on. I usually work my way more-or-less down that list of layers, putting in the "base" terrain last (since, when it's on, it obscures anything else that hasn't been drawn yet). Personally, I generally turn the river layer off after I'm done with it, but leave the other features on while I'm working on the base. Others may have different preferences. :-) As Harlan has observed, the conversion program doesn't do rivers, so you'll need to draw the rivers in a terrain type that will be obviously distinct from the actual surrounding terrain, such as Polar Mountains for most of the world (I switch to Desert when I start getting near the poles, where there actually are Polar Mountains). Remember also that since you're going to resize the final map down to a fairly small size, you need to use a mid-sized brush for them to show up at all.

Once everything is done, you can turn the Original (background) layer on and off, and anything you've missed will show through as white. Once everything really is done, you can delete that layer (if you want), and then prepare for conversion: Flatten the layers, convert to 8-bit palette, and resize to (say) 140x140. When you resize, the resample option you want to use is "Nearest Neighbor" (otherwise, you may end up with, for instance, a fringe of tundra around all your forests). At this stage, you may want to look over the resulting file and fix things, such as connecting any rivers that have been broken in the resize. Run the conversion program and import the text file into ctp2. Go through it to change the Polar Mountains (or whatever you used) to the correct terrain with rivers on them. Get the Continental Shelf around the border between Shallow and Deep water by adding/removing Shallow Water tiles (ctp2 automatically generates Shelf when you do this). Place resources and starting locations, and you're basically done.

There is alway 2 bytes extra.

Rich

As for the error that some people are getting, I've found and probably was told by thousands of others that the map you create must be placed in the text format in the ctp2 directory not the ctp2/save/maps. You must load the map by using the /importmap name.txt function in the chatbox (default button is ' ) As long as the map size you want to import matches one of the possible map sizes defined in the const.txt file it seems to work without a problem (I've had problems of it not importing if it didn't match a size in const.txt... not sure if it's me or if it's the game).