I have lost the original of my photo, which was about 500x500 pixels, JPG. The only one survived copy is 200x200 pixels. So, I want to enlarge it back to original size, and obviously, the main problem is noticeably reduced image quality, i.e. after enlarging 200x200 image back to 500x500 it becomes very blurred.
What tutorials or recommendations I should use to restore (as much as possible) it's original quality, i.e. make it sharp?
Get Smilla image enlarger. It's free. There are also many commercial products for the same purpose - for ex. ON1 Resizer and many others. They are trying to keep sharp and thin lines sharp and thin when enlargening. They recognize jagged edges and make new with finer resolution. Photoshop's image resize doesn't try those things.
My opinion is that Smilla and ON1 Resizer have succeeded. Try them. But know, that those programs cannot guess missing details, altough they can succesfully guess sharp borders.
You will never get the original quality back. It's not possible.
You can try these resizing plugins/techniques others have mentioned, and you might get a fairly decent result. But these can't work miracles. Once you have resized an image smaller, the pixels (and the details) are deleted forever. Resizing larger involves having software re-interpolate the deleted pixels. It has to estimate what the missing detail might have been.
The only real solution is that you should always keep your original files, and never overwrite them when editing. I realise that's probably not what you want to hear, but it is the reality of the situation you face.
First of all, use photoshop to enlarge by using the best for enlargement features.
After that try to sharpen the image by using the high-pass feature (see tutorial here)
If the enlarged image is too pixelated, try adding a very light Gaussian blur and then use the high pass sharpening layer on top of it.
