Create MipMap image manually (iOS OpenGL)

Create MipMap image manually (iOS OpenGL)

Currently I'm loading a texture with this code :
GLKTextureInfo * t = [GLKTextureLoader textureWithContentsOfFile:path
options:@{GLKTextureLoaderGenerateMipmaps: [NSNumber numberWithBool:YES]}
error:&error];
But the result is not that good when I scaled down the image (jagged edged).
Can I create my own mipmap using image software like Adobe Illustrator?
But what is the rule to do that? And how do I load this image using the
code ?
Thanks!