I think you need to mount all images sequentally and then use CTRL-F4 to swap among them. I don't know if this feature is implemented yet, though.

IMGMOUNT A utility to mount disk images and CD-ROM images in DOSBox. IMGMOUNT DRIVE [imagefile] -t [image_type] -fs [imag […]
Show full quote

IMGMOUNT
A utility to mount disk images and CD-ROM images in DOSBox.

IMGMOUNT DRIVE [imagefile] -t [image_type] -fs [image_format]
-size [sectorsbytesize, sectorsperhead, heads, cylinders]

imagefile
Location of the image files to mount in DOSBox. The location is on a
mounted drive inside DOSBox. CD-ROM images can be mounted
directly as well. They don't have to be on a mounted drive.

-t
The following are valid image types:
floppy: Specifies a floppy image or images. DOSBox will automatically
identify the disk geometry ( 360K, 1.2MB, 720K, 1.44MB, etc).
iso: Specifies a CD-ROM iso image. The geometry is automatic and
set for this size. This can be an iso or a cue/bin.
hdd: Specifies a harddrive image. The proper CHS geometry
must be set for this to work.

-fs
The following are valid file system formats:
iso: Specifies the ISO 9660 CD-ROM format.
fat: Specifies that the image uses the FAT file system. DOSBox will attempt
to mount this image as a drive in DOSBox and make the files
available from inside DOSBox.
none: DOSBox will make no attempt to read the file system on the disk.
This is useful if you need to format it or if you want to boot
the disk using the BOOT command. When using the "none"
filesystem, you must specify the drive number (2 or 3,
where 2 = master, 3 = slave) rather than a drive letter.
For example, to mount a 70MB image as the slave drive device,
you would type:
"imgmount 3 d:\test.img -size 512,63,16,142 -fs none"
(without the quotes) Compare this with a mount to read the
drive in DOSBox, which would read as:
"imgmount e: d:\test.img -size 512,63,16,142"

-size
The Cylinders, Heads and Sectors specification of the drive.
Required to mount hard drive images.

An example of CD-ROM images:
1a. mount c /tmp
1b. imgmount d c:\myiso.iso -t iso
or (which also works):
2. imgmount d /tmp/myiso.iso -t iso

Works! Thank you!

You are welcome. 😎