I couldn't find an image converter that could convert both ways, from ST to PC and PC to ST, so I wrote my own.
You can get it from here:http://railtron.com/files/atarist/NEOLoad_V1.3.zip
It only works with 320 x 200 x 16-colour .NEO files and 320 x 200 .BMP files.
To convert from ST to PC, load the .NEO image into the top panel, then save it out as a .BMP..
To convert PC to ST, first resize your image to 320 x 200 and reduce to 16 colours in another paint program, then save as a Windows compatible bitmap. My program won't do any of the scaling or recolouring for you - it will give a warning if it finds more than 16 colours.
Load the .BMP into the bottom window and it will extract the palette. You may find that some of the colours in your original image are combined into the same colour when reduced to the 512 colour palette of the ST, so the palette extracted may be less than 16 colours. You could then try an image with a larger palette, e.g. 20 or 24 colours, or you could try increasing the saturation and contrast in your original image to make the colours more distinct.
You can use the 'Swap Palette' option to re-order the colours in the palette, and 'Preview' to see what the result will look like. Then save out the .NEO version of your image. You can try loading it back into the top again, instead of the Preview.
It was written in Borland C++ Builder 4, and sources are included.