I took some "snapshots" of the process along the way as I was doing one for a weretiger recently. I'll take the lesson about halfway to where I have a decent tiger silhouette, but not go into what I did to "humanize" the tiger outline.
First, I do a Google search for "public domain tiger" and get this fairly clean black & white illustration. This part of the hunt is the most difficult but also the most fun. I check that it's actually in the public domain (the source site, wpclipart.com, is pretty trustworthy).
GIMP 2, which is freeware. I'm still not fully conversant with its use of layers, but know some tricks to get around the annoying aspect of them.
First I copy the tiger from the image, then paste the tiger directly in a new GIMP window from the clipboard, using control-shift-V.
The next step is to clear away the background lines around the tiger. We want to get it to where the tiger outline floats free of the rest.
Next, a little darkening and reduction to plain black-and-white needs to be done, or the silhouette will have gray edges that interfere with the clean black line of the final transparent image. I usually accomplish this by turning the image's "brightness" down and "contrast" up using "Brightness/Contrast" in the Colors menu. Make sure the picture is in RGB format (Image > Mode menu) or this won't work.
We now have a darker-lined tiger with more complete lines. For extra assurance that the image is only black and white use the Posterize command under Colors and ask for only 2 colors. If you get weird colors, the image is not a true monochrome; use "Desaturate" under Colors to get it that way.
Now here's the trick that saves a lot of work. Making sure that the tiger outline is complete, get the bucket fill tool and choose some color neither black nor white to fill the outside area. If there are spaces inside the silhouette, like the gap between an arm and body, those need to be filled too.
Now use the "Select By color" option from the Select menu to grab the red part. Cut it, select all, delete the screen (after returning the color picker to black foreground/white background) and paste it back in, finishing with control-h control-a:
And bucket fill the white with black, then go to Layer > Transparency > Color to Alpha:
Alpha is the channel that makes your image transparent, usually a good feature in a silhouette. Here, you should pick the same color that you used to fill around the outline - in this case, red. That will give you a final silhouette that has a transparent background and can go anywhere.
Caught mid-transformation, with a little shear applied for weirdness, and a couple of ape arms glued on ... anyway, this is how I do 'em!