Regarding image classification using CNN
This means that you are clearly overfitting.
Your model is not able to generalize well to the images that you enter manually.
There are a few things that you could do.

You could try regularization so that your model does not overfit.
Try getting more data
Data augmentation

I hope that by introducing regularization, your model would work better on manually entered images.
I built a image classification model using convolutional neural network for 5 classes.It is giving the training accuracy as 100% and testing accuracy as 82%. But when I gave an image manually for predicting, the model is not able to classify them correctly.For 10 images the model is classifying only 3-4 images correctly.What is the mistake? What should I do??