CCDciel is free software released under the terms of the
GNU General Public License
From the menu File → Open picture file.
This let you load an picture file in one of the following format: png, bmp, jpg, tif, gif, tga.
This support monochrome picture both 8 or 16 bit per pixel, or color picture 3×8 or 3×16 bit per pixel.
The image is converted in 16 bit FITS format to load it in the preview window.
If you want you can then save to a file in FITS format using the menu Save FITS file.
This is useful if you want to review a previous shot from the capture sequence, or to play with some other image.