From the menu File → Open FITS or picture file.

You can also open an image file by drag&drop of the file to the CCDciel window.

This let you load a FITS or picture file in the preview window instead of taking the image with the camera.

This is useful if you want to review a previous shot from the capture sequence, or to play with some other image.

The FITS file must contain a 2 or 3 dimensions image in the primary array. The format of the pixel can be 8, 16 or 32 bit integer, 32 or 64 bit floating point. 3 dimensions array are interpreted as a RGB color image.

DSLR raw file can be any format supported by LibRaw or dcraw.

The picture file format can be one of the following: 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 show in the preview window.
If you want you can then save the picture to a FITS file using the menu Save FITS file.

Note CCDciel is not a multi-purpose FITS file viewer. It is intended to display images from the camera but it could fail displaying FITS files from other programs.
There is also chance CCDciel cannot plate solve a file taken with another software if WCStools cannot find the approximate coordinates and scale of the image.