You should saved resized image along with original image after upload complete.
Say you would save uploaded image in
Images/UserImages then you can save uploaded image's resized version in
Images/Thumbnails. You can resize image to whatever size you want; there are number of articles in codeproject for that.
After that you can write html as
<a href='/Images/UserImages/MyImage.jpg">
<img src='/Images/Thumbnail/MyImage.jpg" />
</a>
So clicking on thumbnail will redirect you to image path from where you can get original size image.
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