Renders or cut outs of fighters are commonly used in most sigs you see on this forum and others, they are pictures that the have been trimmed around the fighter nad then the fighter placed ussually overtop of another series of pictures. This takes a little practice and many begineers reach for the eraser tool which is much more difficult and time consuming than what can be done using the proper tool.
First off grab the pen tool,
Then Zoom in till you have a good view, control+ will zoom in (Control- zooms out) Then click around the edges of what you want to cut out you should get little boxes where ever you click like this.
Continue on around you desired area,you may have to turn the trasparency down on the top layer so you can see it you can do that in the layers menu,
Once you have you fighter completly cut out close the loop by clicking on your original box and right click and select make selection.
Experiment with feathering, personally I like .5 Pixels if your a little rough you may have to go bigger, (Feathering will make the outer edges semi transparent to help blend) If you think you think the outside is to transparent just duplicate the layer (Right click on layer) and play with the transparency of the duplicate. Do this only after positioning and sizing though as other wise it will be to hard to place perfectly.