The best way to check the resolution of your image is in Adobe Photoshop. Open the image in Photoshop and go to Image > Image Size. This will show the width and height of the image (change the units to ‘Centimetres’ if needed) and the resolution (make sure this is set to Pixels/Inch).
Here we have a typical A3 poster design. As we can see, the image has a width of 30.3cm and a height of 42.6cm (this matches A3 with a 3mm bleed). The resolution of the image is 300dpi so this design is set up correctly for print.
This next image has the same width and height, however the resolution is only 72dpi. This means that the image will print at the same size, but have less dots per inch, creating a blurry or ‘pixelated’ image.
Notice the dimensions of the image too. Our 300dpi image has 3579 x 5032 pixels, whereas our 72dpi image only has 859 x 1208 pixels. So even though the image size, in centimetres, is the same on both, the low resolution one has less pixels making up the image.
This third image has a resolution of 72dpi, but notice that the width and height are 126.22cm x 177.48.
The actual size of this image is way bigger than A3, so what we can do is try and resize it.
Resizing an image keeps the same amount of pixels in the design, so Photoshop is not trying to guess what colour pixels to insert like when we resample an image. Rather it will use the existing pixels to resize the image.
The image has 3578 x 5031 pixels. Untick the ‘Resample’ box and change the resolution from 72 to 300. The image below shows the results. The design now has a resolution of 300dpi and a width and height of 30.3cm x 42.6cm, perfect for printing an A3 poster!
Photoshop has not changed the amount of pixels in the design, you can see there are still 3578 x 5031, but it has put more pixels per inch by shrinking the image and increasing the resolution.