I use an application (it's free, btw) called "The Gimp" for my image editing. There is actually a section called "decor" on the program, which gives you the option to click and produce the drop shadow effect. You can control the size of it. I don't know if your image editing software has that option, but I'd guess it does. You might want to do a search once you're in that application, and look for "drop shadow".