Well, there are a lot of options. It really depends on what you have access to, and what you are comfortable using.

Posters should meet the following size conditions:

• Have an aspect ratio of 2:3 (sometimes called 1:1.5). An aspect ratio of 2:3 mean that the height of the poster has to be 1.5 times longer than its width.
• Size between 500x750px and 2000x3000px. So the minimal width for a poster is 500 pixels and the minimal height 750 pixels. The maximal width for a poster is 2000 pixels and the maximal height is 3000 pixels.

So first, you need to understand which condition(s) isn't met.

If the original aspect ratio is not 2:3, you will have to crop (cut) the poster. Cropping is explain in the Tip 5 here. This can easily be done with basic computer apps. For example, you can even do this using the Photo app in Windows 11.

If the original image is smaller than 500x750px, it is unfortunately too tiny and cannot be added. One of the key rule of editing images is that sizing down is fine (as long as proportions are maintained), but images should never ever be sized up. It destroys the quality of the image.

If the original image is bigger than 2000x3000px, you can size it down. You can do this in Windows using Paint. Here's a wikiHow guide explaining how to shrink an image: https://www.wikihow.com/Resize-an-Image-in-Microsoft-Paint Instead of checking the Percentage option at #6, check the Pixels option. Horizontal: 2000 Vertical: 3000

Whenever you are sizing down an image, make sure that the Maintain Aspect Ratio (sometimes displayed as an ∞ symbol) option is checked so the sized down image doesn't end up looking distorted.

Apple seem to have a similar features for Mac users.

Merci beaucoup de votre réponse, finalement j'ai réussi c'était tout bête 👍