I first usually go to Free Online Picture Resizer - Crop and Resize photos, images, or pictures online for FREE! and resize the image I want to use. Then I go to 'User CP' and click on 'Edit Signature'. I hope that helps!
As a fellow novelist, I think a character like that wouldn't be relatable. The age is appropriate, but not the hitman part. If you want to have the main character be something of an antihero that doesn't require too much suspension of disbelief, I'd recommend making him a hardcore hacker, or something along those lines. Best of luck!