Here is another tutorial for paint.net that will show you how to perform a well known effect : the fade edges effect. This is a nice effect for pictures, especially if you want to embed these into a website. Here are the different steps :
1/ Start from the picture to modify;
2/ Add a new layer (Ctrl + Shift + N);
3/ Select the newly created layer and get it down in the layer stack;
4/ Select the top layer (the picture one);
5/ Select (S key) a frame in the central part of the picture (which will not be blured);
6/ Invert the selection (CTRL + I);
7/ Set selection transparency to 100% (adjustements menu > transparency);
8/ Deselect (CTRL + D);
9/ Perform a gaussian blur + (effects menu > blurs > gaussian blur +) with the following values : radius at 25 and choose channel for bluring at alpha channel;
10/ Merge the layers, that’s it!
If you’d like to keep the transparency on edges of the picture so that it fits well with any page background (even if it isn’t white), you need to save it as gif or png and not jpeg as the jpeg doesn’t support the transparency layer.