OpenGL 1.2 or lower subsitute for GL.Ext.GenerateMipmap and GL.GenerateMipmap?

GL.TexParameter(..., GenerateMipmaps, ...) before your TexImage2d call is one option. I'm not sure what version is required for this, however, I think this was introduced in GL 1.4 or so.

For older versions, you need to generate mipmaps manually and load them with GL.TexImage2d using the level parameter. You can either create the mipmap chain offline with Photoshop/GIMP (best quality, larger downloads) or use System.Drawing.Bitmap to downsample the original image at runtime (lower quality, smaller downloads).