How to use bitmap arrays to read & store photos in a specific path

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C#

patterns

image-recognition

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Hi everyone
I'm working on an image processing project these days.
everything seems to be working effectively but suddenly it stopped.
I don't know what the reason is because it was successfully working before.

The idea is that I want the user to open a text file where some information we need is,path chosen by the user also contains 18 jpg photos which program should read and store in a bitmap array previously defined

This is the code:

public partial class Form1 : Form
    {
        Bitmap[] Weapons_imgs = new Bitmap[18];
        double[,] Learning_Vectors = new double[18, 2];

        Rectangle[] Weapons_rects;
        BlobCounter BC = new BlobCounter();
        GrayscaleY Gray_filter = new GrayscaleY();
        Invert negative_filter = new Invert();
        Threshold thre_filter = new Threshold(10);


private void Get_Data_Click(object sender, EventArgs e)
        {
            OpenFileDialog OFD = new OpenFileDialog();
            if (OFD.ShowDialog() == DialogResult.OK)
            {
                string str;
                StreamReader reader = File.OpenText(OFD.FileName);
                string dir = Path.GetDirectoryName(OFD.FileName);
                for (int i = 0; (str = reader.ReadLine()) != null; i++)
                {
                    string[] strs = str.Split(':');
                    Weapons_imgs[i] = new Bitmap(dir+"\\"+strs[0]+".jpg") ;
                    Bitmap img = Gray_filter.Apply(Weapons_imgs[i]);
                    negative_filter.ApplyInPlace(img);
                    thre_filter.ApplyInPlace(img);
                    BC.ProcessImage(img);
                    Weapons_rects = BC.GetObjectsRectangles();

                    double WH = (double)Weapons_rects[0].Width / (double)Weapons_rects[0].Height;
                    Learning_Vectors[i, 0] = WH;
                    Learning_Vectors[i, 1] = double.Parse(strs[1]);

                    Weapon_Features.Series[0].Points.AddXY(Learning_Vectors[i, 0], Learning_Vectors[i, 1]);
                    Weapon_combo.Items.Add(strs[0]);
                }
}
}

when coding it doesn't generate errors but when executing it show me this message:
"

An unhandled exception of type 'System.ArgumentException' occurred in System.Drawing.dll

"

and it points on this line of code:

Weapons_imgs[i] = new Bitmap(dir+"\\"+strs[0]+".jpg") ;

what's the reason of all that?
I don't know what to do because I don't the reason for this

What I have tried:

I've tried to convert the photos to bitmap images and change the line of code to be like:

Weapons_imgs[i] = new Bitmap(dir+"\\"+strs[0]+".bmp");

and I rebuilt the whole project