建议使用方法二
方法一：

//通过图片的URL地址，从网络上获取图片(UIImage类型)
-(UIImage *)getImageFromURL:(NSString *)fileURL{
    
    UIImage * result;
    
    NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fileURL]];
            
    result = [UIImage imageWithData:data];
    //tabbar上的图片需要的尺寸 44x44
    CGSize size22 = CGSizeMake(25, 25);
    UIGraphicsBeginImageContext(size22);
//    CGSize size22 = [UIImage getImageSizeWithURL:[NSURL URLWithString:fileURL]];
//    UIGraphicsBeginImageContext(size22);
    [result drawInRect:CGRectMake(0, 0, size22.width, size22.height)];

    UIImage *newImage2 = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    result = newImage2;
    
    return result;
    
}

方法二：

//导入头文件
#import "SDImageCache.h"

// 加载图片
- (void)loadImage:(NSString *)image complet:(void(^)(UIImage *img))complet {
    // 设置图片 获取本地缓存的图片
    UIImage *newImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:image];
    if (newImage != nil) {
        // newImage就是图片 压缩尺寸25*25
        UIImage *newImage1 = [self imageCompressForSize:newImage targetSize:CGSizeMake(25, 25)];
        complet(newImage1);
    } else {
        //下载图片
        [[SDWebImageManager sharedManager] loadImageWithURL:[NSURL URLWithString:image] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
            
        } completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL) {
            if (image) {//下载完成后
                //同上处理
                UIImage *newImage = [self imageCompressForSize:image targetSize:CGSizeMake(25, 25)];
                complet(newImage);
            }
        }];
//        [[SDWebImageManager sharedManager] downloadImageWithURL:[NSURL URLWithString:image] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
//
//        } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
//            if (image) {//下载完成后
//                //同上处理
//                UIImage *newImage = [self imageCompressForSize:image targetSize:CGSizeMake(25, 25)];
//                complet(newImage);
//            }
//        }];
    }
}

//图片压缩色值不变
- (UIImage *)imageCompressForSize:(UIImage *)sourceImage targetSize:(CGSize)size{
    
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = size.width;
    CGFloat targetHeight = size.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0, 0.0);
    
    if(CGSizeEqualToSize(imageSize, size) == NO){
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if(widthFactor > heightFactor){
            scaleFactor = widthFactor;
        } else{
            scaleFactor = heightFactor;
        }
        scaledWidth = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        if(widthFactor > heightFactor){
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }else if(widthFactor < heightFactor){
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    
    UIGraphicsBeginImageContextWithOptions(size, NO, 3.0);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    if(newImage == nil){
        NSLog(@"scale image fail");
    }
    UIGraphicsEndImageContext();
    return newImage;
}