A very short video to get you started with creating your own worms levels using the free Gimp image editor (available here).

The video can be viewed in Firefox or Chrome:
Video

There are no comments so here are the written instructions (it's also pretty much impossible to keep up, so use that pause button):

*Create a new image with File/New
*Set size to 960x400
*Open advanced and set Fill With to Transparency
*Start drawing. Remember to use a sharp brush for the outline as worms only allows for either transparent or opaque pixels, nothing in-between
*Choose Image/Mode/Indexed
*Choose Optimum Palette, 79 Colors, disable dithering of transparency. Set dithering to Positioned (you can try the other modes too, but positioned or disabled usually look best)
*Choose Image/Mode/RGB
*Set the background color to something exotic. I'm using magic pink (255/0/255) here. The color shouldn't appear anywhere inside your image.
*Choose Layer/New Layer
*Set Layer Fill Type to Background Color
*Drag the pink layer below your original one
*Choose Image/Flatten Image
*Choose Image/Mode/Indexed
*Set Maximum number of colors to 80
*Choose Colors/Map/Rearrange Colormap
*Drag your transparent color to the first slot
*Choose File Save As
*Go to the DATA/CUSTOM/GFX folder inside your worms directory
*Enter a name and make it end in .PCX (only use 8 alphanumeric chars in front of the .PCX)
*Find the worms link in your start menu
*Right-click and choose Properties
*Remove -c "exit" from the commandline
*Open notepad
*Open dosboxWORMS.conf in your worms directory
*Scroll to the bottom and put a # in front of call WORMS and exit
*Save
*Open the worms link
*On the commandline enter (where MYLEVEL.PCX is the filename you used while saving and PLAYLVL.PCX is another 8 char filename that will be used for the level name):
cd data
cd custom
cd gfx
gfxcon MYLEVEL.PCX PLAYLVL.PCX
*You can now start the game again
cd \
worms
*To play the level hit space on the level select screen and enter the filename you used for gfxcon.

Now I'm having this problem where GIMP locks up whenever I try to save or open a file. BLARGH. Must find better alternative.

Never seen that one before. While I despise the traditional Gimp interface, it is probably more stable than anything else you can find. You could use the RGB editing solution of your choice and convert the result with ImageMagick to PCX.

My question is how exactly you can edit the two BK files?

"The video can be viewed in Firefox or Chrome"

or Opera

Kinda sad to see that there was no answer to my question six years prior, LOL. Back then I had managed to create a few maps, mostly based on sprites from popular games and even back then had struggles with those pesky BK files. Sadly my hard drive bit the dust, the maps were lost and I lost quickly interest. Strangely I am here again and interested in making maps again to ask a bit of a different question, yet similar to the one before: Is there easier way to mark the main color the BK1 file (converted as a PCX) as transparent? Tried with GIMP, it works only for the main PCX 960x400 map file after going through GFXCON. The BK2 (the second background) file is not visible at all and the would be transparent color of the BK1 file covers the whole background (screen attached). Any ideas how to overcome this issue?