Hi Joane,
One of the more common causes of blurry images in a Storyline 360 course is mismatch between the resolution of the added image and the slide size of the project. The media library will give you a quick overview of the resolution of the images that you have in your course. You can also check out the article below to see our list of best practices to avoid the loss of media quality:
If the resolution of your images isn't too far off from the size of your slides, would you be willing to upload a copy of your project file here so we can take a look at what's happening?