If you've spent any time learning how to modify photos, you probably already know that the clone tool is your best friend.

The great thing about photographs that most people don't realize is that even tho it is a flattened image that seems difficult to work with, photos are just repeatable color and texture patterns.  The thing that holds most people back from doing quality photochops is when they attempt to 'fill in the gaps' or replicate something that is in another part of the photo, while changing key parts to make it a 'chop'.  The human brain is so used to seeing what I call "real life colors" that they instinctively assume that a wall is white, or the road is blackish grey, but in fact, a solid color very rarely occurs in nature, or reality for that matter.  The closest you will ever get to seeing an area of a single color in a photo would be if there was a glossy, well-lit poster or photograph IN the photo itself, and even in that case there would be shade variations depending on the nature of the light hitting it.  All natural looking colors in photographs are actually a textured pattern consisting of many shade and hue variations of a particular color, giving it that true to life look.

While the technology to simply generate natural looking textures that blend well with photos is virtually impossible, most of these color patterns can be replicated using the clone tool. Using any size brush with a very low hardness ratio is ideal for recreating patterns and textures that can be used to cover up or add to particular elements in a photo.

Here's an example:

This is the original image from Serial Experiments of Lain

Using the clone tool I carefully replicated the vertical lined background over the top of "Lain".
A good tip here is to replicate from one layer onto a new layer, so that you can hide the layer and go back to the original document for comparison, or if you mess up, but don't delete layers! The first one you make might end up being the best you can do! Trust me on this one ;)

The final step is a matter of matching the theme the best you can.  Remember, most fonts used on signs, posters, business cards, etc. are font that are publicly available for download!  It takes a FANTASTICALLY large ammount of effort to create a font, and very few people care to try just so they can use an original font in their ads or whatever.
In this particular case the font IS original because the animation was drawn by hand, not a computer, so the font will be virtually impossible to match perfectly.  I used 'Modern' which looked relatively similar, and since none of the original login name remains, there's no reason to match it perfectly.  I copied and pasted the cursor square from the original image and reduced the opacity of 'Pak Behl' slightly to match the color and nature of the square, then I used a gaussian blur to blur 'Pak Behl' since the original image isn't very sharp so a sharp font would look like it doesn't belong. I then created a drop shadow and matched it's features to what I saw from the original. And I also added a black inner glow (FireWorks Effect) which gives the font a multitude of light and darker shades of blue like the original does.
It is mostly a matter of practice and experience when it comes to matching fonts and effects because you never know how the original artist did it, but after a while you learn to recognize certain effects and fonts so and you'll just get better and faster at doing it. It doesn't take too long if you keep at it ;)

Have fun with this, and post what you come up with!!