I have two images with different size and resolution. I need to do a cognitive experiment so is mandatory that different images are as much equal as possible. Is there any method to reduce the quality of an image in terms of size and ppi to make it equal to the other one?
You can scale an image to a smaller size and allow your raster image editing software to "resmaple" it. You can do it using almost any raster image editor.
Note that the resolution in PPI/DPI is not the problem here. The true resolution of a digital image, and its quality is only dependent on its size in pixels. So, you should check the size in pixels to make both images the same or similar sizes. You can ignore the resolution settings, they aren't really relevant here. The PPI/DPI might be relevant if you are using the images to layout a document for print, but that's a different issue. If you don't understand resolution and what it is/isn't, then for further reading check The Myth of DPI.
Here's an example using the popular free image editor GIMP (or Photoshop which isn't free), but this would be possible in other software too.
- Open an image, using File > Open
- Click Image > Scale Image (the equivalent in Photohsop is Image > Image Size)
- Type in either a new width or new height in pixels. The software will automatically work out the other dimension so the image is not distorted, i.e. so that the aspect ratio is preserved.
Note: in other software such as Photoshop, you may need to check an option that says something like "resample". You don't have to do this in GIMP.
- Finally click Scale, and File > Export as (or Save as in Photoshop), to save your image in the required format.
Warning when exporting an image, make sure that you use a new file name or you may overwrite the original image file. If you do that, then it can't be undone!
Example showing GIMP's Scale Image dialog
Example showing Photoshop's Image Size dialog.