支持网络和本地图片压缩
实现思路：先压缩图片的大小，再压缩图片的质量。
网络图片转image对象:

NSData *temdata = [NSData dataWithContentsOfURL:[NSURL URLWithString:@"图片链接地址"]];
UIImage *image = [UIImage imageWithData:temdata];

data转image：

self.imageView.image = [UIImage imageWithData:data];

图片压缩代码完整实现如下：
@param image 图片对象
@param maxLength 最大体积KB为单位
@return 压缩后的图片data数据
压缩图片至最大体积为maxLength

+ (NSData*)compressImageQuality:(UIImage*)image toByte:(NSInteger)maxLength {

    //先压缩size,再压缩质量
    UIImage *newimage = [self thumbnailWithKeepScale:image size:CGSizeMake(150, 150)];
    NSData *data = UIImageJPEGRepresentation(newimage, 1.0);
    if (data.length>maxLength*1024) {
        UIImage  *image1 = [self compressImage:newimage toByte:maxLength];
        NSData *data = UIImageJPEGRepresentation(image1, 1.0);
        return data;
    }else {
        return data;
    }
}

根据最新的size，保持原图的宽高比，生成一个缩略图

+ (UIImage *)thumbnailWithKeepScale:(UIImage *)image size:(CGSize)asize
{
    UIImage *newimage;
    if (nil == image) {
        newimage = nil;
    }
    else{
        CGSize oldsize = image.size;
        CGRect rect;
        if (asize.width/asize.height > oldsize.width/oldsize.height) {
            rect.size.width = asize.height*oldsize.width/oldsize.height;
            rect.size.height = asize.height;
            rect.origin.x = 0;
            rect.origin.y = 0;
        }
        else{
            rect.size.width = asize.width;
            rect.size.height = asize.width*oldsize.height/oldsize.width;
            rect.origin.x = 0;
            rect.origin.y = 0;
        }
        UIGraphicsBeginImageContext(rect.size);
        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextSetFillColorWithColor(context, [[UIColor clearColor] CGColor]);
        UIRectFill(CGRectMake(0, 0, rect.size.width, rect.size.height));
        [image drawInRect:rect];
        newimage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }
    return newimage;
}

循环压缩图片质量

+ (UIImage *)compressImage:(UIImage *)image toByte:(NSUInteger)maxLength {
    CGFloat compression = 1;
    NSData *data = UIImageJPEGRepresentation(image, compression);
    if (data.length < maxLength) return image;

    CGFloat max = 1;
    CGFloat min = 0;
    for (int i = 0; i < 6; ++i) {
        compression = (max + min) / 2;
        data = UIImageJPEGRepresentation(image, compression);
        if (data.length < maxLength * 0.9) {
            min = compression;
        } else if (data.length > maxLength) {
            max = compression;
        } else {
            break;
        }
    }
    UIImage *resultImage = [UIImage imageWithData:data];
    if (data.length < maxLength) return resultImage;
    NSUInteger temLength = 0;
    while (data.length > maxLength && data.length != temLength) {
        temLength = data.length;
        CGFloat ratio = (CGFloat)maxLength / data.length;
        CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)),
                                 (NSUInteger)(resultImage.size.height * sqrtf(ratio))); 
        UIGraphicsBeginImageContext(size);
        [resultImage drawInRect:CGRectMake(0, 0, 100, 100)];
        resultImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        data = UIImageJPEGRepresentation(resultImage, compression);
    }

    return resultImage;
}