Are you dragging them in from the file list in WorldEd or the file browser on your computer? I forgot to mention I could only get them into the editor if I dragged from the Windows file explorer. Dragging from the file list on the right side of WorldEd only seems to work for adding buildings in my experience. Also just to double-check, are you sure the images are the right resolution? They should be multiples of 300 since a 300x300 pixel square is a single cell in WorldEd. I haven't tried it, but it might not let you import it at all if it's the wrong resolution. Or maybe you can't import it if the image would require more cells than you set up your world for. You could always try using the World -> Resize World menu to increase it to make sure the .bmp will fit when imported. You might also need to have the image files in the same directory as your world's .pzw file, but I'm not certain on that. That's how I have mine set up at least.
The only other thought I have is to try re-exporting your images from your image editor in different formats. Either different bit-depth .bmp files or try .png. Personally, I use .png with max compression since it doesn't lose any quality while saving a lot of disk space, and I haven't had any import issues with them. If you're still having issues, I can put together a basic template world and blank base/vegetation maps to see if you can at least load an existing world file.
On the editing existing cells side, from what I've read around the forums, I believe you have to completely replace an existing cell instead of modifying an existing one. So you can use the online map at https://map.projectzomboid.com/ to find the coordinates of the cells you want to edit and to use as a reference when making your replacement cells. Unfortunately as far as I can tell you'll have to rebuild the existing cells and buildings from scratch even if you just want to slightly change them. Once your cells are finished (or at least ready for testing) you can follow the "Exporting and Testing the Map" section of the official guide here:
and when you get to the Generate Lots menu, make sure you enter the top-leftmost coordinate of the cells you want to replace in the World Origin box so the top-leftmost cell of your new cells will be placed properly. If you leave it set to the default of X: 0, Y: 0, it'll put your new cells in the far northwest corner of the map. And don't forget to add the "lots=Muldraugh, KY" line mentioned in the guide above to your map.info file so the game knows to overlay your new cells onto the base map instead of considering them their own little world.
