I do not know how you will get your pictures from My Gun DB but I am assuming that they are exported somehow. So, this is how you can import those pictures into NM Collector. Please see screen captures below.
When you manipulate the csv file to import data into NM Collector, make sure you add unique (please, no duplicates!) sequential numbers in the NMCSWITEMID column. Then, add a folder containing pictures of each item in the same directory as your csv file. Name the folder according to the NMCSWITEMID in your csv file. So, if your gun with an item number of 3 has pictures, create a folder named Firearms3 and put your pictures for that item in that folder.
Here are the screen captures:
Sample data export directory contents:
Contents of csv file with sample NM Collector Data:
Adding pictures folders for firearms 2 and 3:
This will import your data into NM Collector Software.