SkinShifter
A Minecraft 1.7.10 mod that allows players to change their skin to arbitrary images. It also supports multiplayer.

SkinShifter is a very easy-to-use Minecraft mod that allows players to instantly change their skin to any custom texture in-game. In multiplayer, other players immediately see the change as well. As such, SkinShifter is the perfect tool to use when recording Minecraft movies with many different characters, saving you precious filming time that would otherwise be wasted with painfully long skin-changing intervals. No longer does anyone need to go to Minecraft.net to change their skin!

You can find the source code and contribute to the source repository on GitHub.

This documentation for SkinShifter is split into three pages. The first one shows you how to install the mod, the second one shines some light on how you can load your custom skin files into the mod, and the third one depicts the very simple process of actually selecting the skin and wearing it.