There were a ton of instructions (which I figured out myself) that were on the old site that weren't listed here, so I'll paste them below.

My single addition, which is CRITICAL, is that it will NOT work until after a server restart due to the server (not just the plugin) needing to load the libGDI+ files. 

So if you've done EVERYTHING else possible, restart your Rust server. 

Help! All pictures are white after loading an image from a website!
Because of recent changes to the game images need to be a specific width and height that doesn't always match the sign. To make it easier for the user to upload an image the plugin handles the resizing. This however added an additional requirement for the plugin, gdi+. This library is normally installed on Windows machines but usually missing on Linux distributions. If the image is appearing white it means libgdiplus is missing and you need to install it on the server machine.

How do I install libgdiplus on my machine?
Firstly you need to add the Mono repository to your system, you can do this with the following commands.

Ubuntu 16.04

Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/ubuntu xenial main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
Ubuntu 14.04
Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/ubuntu trusty main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
Ubuntu 12.04
Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/ubuntu precise main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
Debian 9
Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/debian stretch main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
Debian 8
Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/debian jessie main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
Debian 7
Code:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/debian wheezy main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
CentOS 7
Code:
yum install yum-utils
rpm --import "http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF" 
yum-config-manager --add-repo http://download.mono-project.com/repo/centos7/
CentOS 6
Code:
yum install yum-utils
rpm --import "http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF" 
yum-config-manager --add-repo http://download.mono-project.com/repo/centos6/
After adding the required repositories you can then install gdilibplus by running the following commands.

Ubuntu, Debian
Code:
sudo apt-get install libgdiplus
CentOS
Code:
yum install libgdiplus