While reading forums I encountered several times user problems with using disc image support in DOSBOX (great feature made by CANADACOW which enables to use alternative versions of DOS). Therefore I provide this practical sample which worked for me (I am not replacing README here - see there for details about commands IMGMOUNT and BOOT).
The following was tested using bootdiscs for MS DOS 5, MS DOS 6:
1. I created a 10 MB disc image using bximage.exe utility from Bochs http://bochs.sourceforge.net (below are details):
Do you want to create a floppy disk image or a hard disk image?
Please type hd or fd. [hd] hd
Enter the hard disk size in megabytes, between 1 and 32255
I will create a hard disk image with
sectors per track=63
total size=9.84 megabytes
What should I name the image?
Writing:  Done.
I wrote 10321920 bytes to (null).
The following line should appear in your bochsrc:
ata0-master: type=disk, path="c.img", cylinders=20, heads=16, spt=63
2. I put the HD image (c.img) together with an image of a bootable floppy (msd60.ima) into the directory d:\dosbox on my disc.
3. Then I went on with creating of a patition on the disc. I started DOSBOX with the following lines:
mount d: d:\dosbox
imgmount 2 d:\c.img -size 512,63,16,20 -fs none
4. After DOS booted I started FDISK and created a primary DOS partition. When FDISK wanted to reboot later, it crashed/froze (depending on DOS version). I ignored it and ended DOSBOX (it is minor nuisance but FDISK already did its work).
5. I again restarted DOSBOX as in the point 3., but instead of FDISK I used: FORMAT C: /S
This formatted the disc and made it bootable.
6. Now the disc was usable - either for booting or when mounted from DOS shell.
For booting I did this:
mount d: d:\dosbox
imgmount c d:\c.img -size 512,63,16,20 -fs fat
boot -l c
WARNING: Do not copy files to harddisc image from DOSBOX shell when using DOSBOX 0.63 or lower version. It corrupts the image !!! This was fixed in CVS - so use CVS build if you want to do it.
If you need to get some files to/from the disc images, you can use the DiskExplorer available on http://hp.vector.co.jp/authors/VA013937/editdisk/index_e.html
(choose VMWare format).
If somebody is wondering why I did not specify parameter -t for IMGMOUNT: I did not have to, its default value is "hdd".
The parameter -fs has also a default value ("fat") - so "-fs fat" was also not needed.