@Flavio The simplest solution is to download the image from somewhere else in hopes that it is both a regular high density floppy disk and a raw sector file (.img, .dsk, etc.), then write the disk back using WinImage as suggested above.
However, if there is no alternative file, or, the disk image is a 400K or 800K disk image, there are a few options available to you. The
free way is to download a copy of
Basilisk II, download this custom
Disk Image I made for you, and the
IIci.rom file. The Disk Image I made has System 7.1.1 Pro, Stuffit Expander, and DiskCopy 6.3.3. You'll need these to convert a Macintosh .img file.
Configure Basilisk II to use the Disk Image I provided (and set it the System to Macintosh IIci with a 68030 + FPU) and set a directory for the "UNIX" directory. Next, download a copy of
convert2dsk.
Copy the .sit or binary encoded .img file to your emulator via the "UNIX" shared drive. Launch Basilisk II, copy it to your virtual Mac's hard drive, then decode it with Stuffit Expander. Inspect the .img file by selecting it, then Get Info via the File menu. If it is roughly 2MBs, then just open Disk Copy 6.3.3 and do a "Convert Disk Image," select the file, then save it as a "Disk Copy 4.2 Image." Copy the image to the shared UNIX folder. Open convert2dsk, convert the disk image. You now have a raw sector disk image ready to use in any imaging software, emulator, or drive emulator.
If it is a 400K or 800K disk, mount the disk first using Disk Copy. Then "New Disk Image from Disk," select the mounted disk, change the file size to standard 1.4MB, and set the format as Disk Copy 4.2. Save, copy to UNIX drive, convert using convert2dsk.
However, if you intend to use these files on a real Macintosh, then buying a
FloppyEmu is a worthy investment. It is a file based Apple disk emulator useable on both Macintosh and Apple II computers with a 3.5" drive support. It also supports the Disk Copy 4.2 format without the need to convert it to raw sector image. In addition to raw sector images and DC4.2 images, it also now supports the new MOOF format, enabling you to use copy protected software and games.
With a FloppyEmu attached, you can mount the image files directly to your Mac's desktop exactly like a floppy disk. You can then duplicate the disk using Disk Copy to the physical floppy drive, and have yourself an actual floppy. With the SD card, it makes it easy to copy disk images back and forth between your PC and vintage Mac.