I've been using Gimp for quite some time now, and it may not be as powerfull as photoshop but currently it's the best possible alternative to Photoshop you could find. Gimp is Open-source photo editing tool and is versatile in nature as it supports python scripting which allows users to add additional functionality to it.
I've made a quick tutorial on how to add an Inner Shadow effect with Gimp
Here are the steps to get it going.
1) First make a selection around the image with a 5-10px gap.
2) Invert the selection.
3) Open the drop shadow tool from the filters menu and change the values to
Offset X = 0
Offset Y = 0
Blur radius = 30px
color = #000000(black)
Opacity = 75
Allow resizing = Unchecked
and click on Ok.
4) Deselect all (Shift+Ctrl+A).
5) Make a selection around the inner shadow edges and crop to selection.
Here's a screencast.
You may want to adjust the settings to your needs.