Lol MSPaint.

Microsoft Paint
Adobe Photoshop
Paint Shop Pro
Paint.NET
Gimp

Open HLMV (half life model viewer) go to textures tab,select the arm texture and click on IMPORT TEXTURE and look for the hand skin you want to install.Then save the model

If you're installing them into the game, you need to use a model viewer like Jed's Half-Life model viewer, load the model you want the hands on, click the texture tab, find the hands, click import, select the .bmp, preview it in the model display tab, and, if it worked, save the model over the original, backing the original up if you please.