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 IRIS image format) PNG, BMP and NetPBM binaries for images, AVI for films.