通过测试的系统：
Windows 8.1 64bit
Ubuntu Server 16.04 LTS 64bit
Fedora 24 64bit
CentOS 7.2 64bit可以实现以下功能:
Open jpg, bmp, ico, png
Save jpg, bmp, ico, png
Resize image
Draw graphics with brush and pen
Open font and draw string

int codeW = 80;
int codeH = 30;
int fontSize = 16;
Random rnd = new Random();
//颜色列表，用于验证码、噪线、噪点
Color[] color = { Color.Black, Color.Red, Color.Blue, Color.Green, Color.Orange, Color.Brown, Color.Brown, Color.DarkBlue };
//字体列表，用于验证码
string[] font = { "Times New Roman" };
//验证码的字符集，去掉了一些容易混淆的字符 //写入Session、验证码加密
//WebHelper.WriteSession("session_verifycode", Md5Helper.MD5(chkCode.ToLower(), 16));
//创建画布
Bitmap bmp = new Bitmap(codeW, codeH);
Graphics g = Graphics.FromImage(bmp);
g.Clear(Color.White);
//画噪线
for (int i = 0; i < 1; i++)
{int x1 = rnd.Next(codeW);int y1 = rnd.Next(codeH);int x2 = rnd.Next(codeW);int y2 = rnd.Next(codeH);Color clr = color[rnd.Next(color.Length)];g.DrawLine(new Pen(clr), x1, y1, x2, y2);
}
//画验证码字符串
for (int i = 0; i < chkCode.Length; i++)
{string fnt = font[rnd.Next(font.Length)];Font ft = new Font(fnt, fontSize);Color clr = color[rnd.Next(color.Length)];g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * 18, (float)0);
}
//将验证码图片写入内存流，并将其以 "image/Png" 格式输出
MemoryStream ms = new MemoryStream();
try
{bmp.Save(ms, ImageFormat.Png);return ms.ToArray();
}
catch (Exception)
{return null;
}
finally
{g.Dispose();bmp.Dispose();
}

apt-get install libgdiplus
cd /usr/lib
ln -s libgdiplus.so gdiplus.dll

dnf install libgdiplus
cd /usr/lib64/
ln -s libgdiplus.so.0 gdiplus.dll

yum install autoconf automake libtool
yum install freetype-devel fontconfig libXft-devel
yum install libjpeg-turbo-devel libpng-devel giflib-devel libtiff-devel libexif-devel
yum install glib2-devel cairo-devel
git clone https://github.com/mono/libgdiplus
cd libgdiplus
./autogen.sh
make
make install
cd /usr/lib64/
ln -s /usr/local/lib/libgdiplus.so gdiplus.dll