In this lesson we show you how to enlarge a photo (of decent quality) to make a wallpaper.

We use the open source GIMP program that you can download for free here:

We downloaded a photo of a beautiful picture of flowers, which we found on the website of the state museum (Rijksmuseum) in the Netherlands and we use this photo for the tutorial.



Open the program and go in the menu to 'File' and then 'Open'.

Find the photo you want to use and click 'Open' (or click on it twice).

If the photo does not have the right proportions, you can select a part and use only that one. Let's see how.

To select a part click on the 'square' tool (or go to the menu under 'Tools', 'Selection Tools', 'Rectangle Select') and drag over the part you want to keep. Under the tools you see that you can enter values. Obviously you can not enter the measurements of the wall, because the photo is not (yet) big enough. So from the wall measurements you can see the right proportion and calculate what you need to insert.

Then you have to copy the part of the photo you selected. You can use keys (Control C on windows or command c on mac) or go in the menu on 'Edit' and 'Copy'.

Now we open the copied part in a new document using the keys 'control shift v' for windows or 'command shift v' for mac. You can also achieve the same result by going to the menu to 'File' - 'Create' - 'From Clipboard'.

Let's see how big the photo is in the right proportions. To print the wallpaper, which is not made to see from very close by, just 100 dpi is enough for a good quality (which means one hundred dots per inch). Go to the menu  and select 'Image' - 'Print Size'. In the window, you can see some data. Set to see the width and height in mm or cm. X and Y resolution can go to 100 dpi. If your photo has only 72 dpi, you can leave those. If the photo has more than 100 dpi, you will see that the measurements written in Width and Height increase when you enter 100 dpi. Click OK.

Now we have come to the true enlargement of the photo. Obviously the better the photo, the better the result.

Go back to the menu and select 'Image' - 'Scale Image'. Change to mm or cm and make sure the link between width and height remains intact (see video). Enter the width of the wall in 'Width' and if you have selected the right proportion of the photo, the height changes to the right height. If not, you have to start from the selection again. We use 'Quality Interpolation' 'Zinc Lanzarote' and click on 'Scale'. Now probably GIMP asks you to confirm that you really want to make such a large document and click 'Scale' again. It takes a little patience, underneath there is a progress bar that slowly flows to the right.

Now we have come to the last part ... checkin the enlarged photo. To control it well, you have to see it at 100%. Click on '+' to enlarge and on '-' to zoom out, or select 100% on the bottom left. If you use the keys, you can always see which % you are at, at the bottom left.

Check the photo to see if the result is OK and if it's OK, you have to save the photo. The easiest way is to export it to jpeg. Click on the 'File' menu and then 'Export As' and write the file name and select where you want to save. Click 'Export'. A little window  opens and you have to export at 100% of the quality.

You can also save the file in the GIMP format if you think you still want to change things. In the 'File' menu go to 'Save'.