To process your images with Siril, you must convert them to the FITS format it uses (16-bit unsigned, bottom-top order, 1 or 3 axes). Fortunately, Siril is able to convert some image formats to this format, RAW, TIFF, JPEG, PIC (Christian Buil's IRIS image format) PNG, BMP and NetPBM binaries for images, AVI and other films.