You can use WinImage 8.5 Professional to create .mmc image file for proteus. Download and install WinImage 8.5.
Run WinImage and Choose File>New
In the dialog box's Format and Imported section choose "Select Custom Image Format" and click OK. In the next dialog box choose Fat12/16 for FileSystem and click OK.
It will ask to save file. give some name for the file and save it as .IMA format. Then Go To menu Image>Inject and in the dialog box select the files to be injected and click open. Files will be injected. Now click save button and exit WinImage. Now rename the .IMA file you created with .mmc extension. you can use the .mmc file with proteus.