我使用像下面这样的功能来获取任何大小的图像。
原始图像应该比你想要的要大（ps：你可以尝试一个图像）

  +（UIImage * ）thumbnailWithImageWithoutScale：（UIImage *）图像大小：（CGSize）wantSize 
 {
 UIImage * targetImage; 
 if（nil == image）{
 targetImage = nil; 
} else {
 CGSize size = image.size; 
 CGRect rect; 
 if（wantSize.width / wantSize.height> size.width / size.height）{
 rect.size.width = wantSize.height * size.width / size.height; 
 rect.size.height = wantSize.height; 
 rect.origin.x =（wantSize.width  -  rect.size.width）/ 2; 
 rect.origin.y = 0; 
} else {
 rect.size.width = wantSize.width; 
 rect.size.height = wantSize.width * size.height / size.width; 
 rect.origin.x = 0; 
 rect.origin.y =（wantSize.height  -  rect.size.height）/ 2; 
} 
 UIGraphicsBeginImageContext（wantSize）; 
 CGContextRef context = UIGraphicsGetCurrentContext（）; 
 CGContextSetFillColorWithColor（context，[[UIColor clearColor] CGColor]）; 
 UIRectFill（CGRectMake（0，0，wantSize.width，wantSize.height））; //清除背景
 [image drawInRect：rect]; 
 targetImage = UIGraphicsGetImageFromCurrentImageContext（）; 
 UIGraphicsEndImageContext（）; 
} 
 return targetImage; 
} 
 

i am creating app using facebook. if i am trying to upload photo to facebook means i got following message any give idea for solve that

"The provided user_generated photo for an OG action must be at least 480px in both dimensions"

I use a function like follow to get an image with any size. Original image should big than you wanted.(ps:You can try an image little)

+ (UIImage *)thumbnailWithImageWithoutScale:(UIImage *)image size:(CGSize)wantSize
{
    UIImage * targetImage;
    if (nil == image) {
        targetImage = nil;
    }else{
        CGSize size = image.size;
        CGRect rect;
        if (wantSize.width/wantSize.height > size.width/size.height) {
            rect.size.width = wantSize.height*size.width/size.height;
            rect.size.height = wantSize.height;
            rect.origin.x = (wantSize.width - rect.size.width)/2;
            rect.origin.y = 0;
        } else{
            rect.size.width = wantSize.width;
            rect.size.height = wantSize.width*size.height/size.width;
            rect.origin.x = 0;
            rect.origin.y = (wantSize.height - rect.size.height)/2;
        }
        UIGraphicsBeginImageContext(wantSize);
        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextSetFillColorWithColor(context, [[UIColor clearColor] CGColor]);
        UIRectFill(CGRectMake(0, 0, wantSize.width, wantSize.height));//clear background
        [image drawInRect:rect];
        targetImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }
    return targetImage;
}