2d170-quickselect1 On GIMP I often use 'quickmask'. On bottom left corner of the picture area on GIMP there is so called 'quickmask' button. To use it, make sure you select all: 'Select > All'. Then click the quickmask button. Now the picture should turn into reddish mask. (For some reason the image does not always turn reddish when mask becomes applied. Do not let that bother but continue to the next stage.)

abac0-quickselect2 Next stage is to decide what you select, and for this you can use 'Blend tool' with different 'Gradients'. Select for example 'Radial Gradient' to treat the edges differently than the center of the image. When you are done you get the reddish mask blending gradually from darker to lighter red.

09a20-quickselect3 Next you need to click the quickmask button again, and the mask turns into selection. Now make sure you get selected what you want. If only the border of circle flickers, then circle is what you get. But if you choose 'Select > Invert', then it is the area between the circle and edges of the image selected.

8631f-serola20130120a Finally you can for example duplicate the layer, run 'Filters > Artistic > Photocopy…' for the edge part of the image, and finally turn the overlapping layer to 'Soft light' layer mode. As a result you get nice white vignetting (edges are lighter than center of the image).