Using the Gravatar API in a Cocoa Application: http://blog.indragie.com/post/5122436268/using-the-gravatar-api-in-a-cocoa-application

gistfile1.m

@interface NSString (Additions)
- (NSString*)stringByRemovingWhitespace;
@property (readonly) NSString *MD5;
@end

@implementation NSString (Additions)

- (NSString*)stringByRemovingWhitespace
{
    NSCharacterSet *whitespaces = [NSCharacterSet whitespaceCharacterSet];
    NSPredicate *noEmptyStrings = [NSPredicate predicateWithFormat:@"SELF != ''"];
    NSArray *parts = [self componentsSeparatedByCharactersInSet:whitespaces];
    NSArray *filteredArray = [parts filteredArrayUsingPredicate:noEmptyStrings];
    return [filteredArray componentsJoinedByString:@" "];
}

// From <http://stackoverflow.com/questions/2018550/how-do-i-create-an-md5-hash-of-a-string-in-cocoa>
- (NSString*)MD5
{
    const char *cStr = [self UTF8String];
    unsigned char result[16];
    CC_MD5( cStr, strlen(cStr), result );
    return [[NSString stringWithFormat:
            @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",
            result[0], result[1], result[2], result[3], 
            result[4], result[5], result[6], result[7],
            result[8], result[9], result[10], result[11],
            result[12], result[13], result[14], result[15]
            ] lowercaseString];  
}
@end

gistfile2.m

static NSString* const kGravatarAPIRoot = @"http://www.gravatar.com/avatar/";

+ (NSImage*)avatarWithEmail:(NSString*)email size:(CGFloat)size defaultImage:(NSString*)def rating:(NSString*)rating error:(NSError**)error
{
    if (!email) { return nil; }
    NSMutableString *requestURL = [NSMutableString stringWithFormat:@"%@%@", kGravatarAPIRoot, [email stringByRemovingWhitespace].MD5];
    if (size) { [requestURL appendFormat:@"?s=%d&", (int)size]; } 
    if (def) { [requestURL appendFormat:@"?d=%@&", def.URLEncodedString]; }
    if (rating) { [requestURL appendFormat:@"?r=%@", rating]; }
    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:requestURL]];
    NSData *imageData = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:error];
    [request release];
    return [[[NSImage alloc] initWithData:imageData] autorelease];
}