If I found image that's not watermarked does that mean that i can use it safely? and what's the relation between using copyrighted images and my ranking aspects?
Using non watermarked images doesn't guarantee that it is not copyrighted and you can use it safely. Also using either the copyrighted image or not may not have any effect on your SEO rankings.
Simply copying an image because there is no watermark is no guarantee that the image does not have an associated copyright. While many stock photo companies use watermarked images there are several that dont but the images still have copyright. Also licencsing of images may be different depending on the intended use of an image, licencing fees being higher for more flexible usage. As for ranking, copyrighted and non-copyrighted material is not differentiated and so will not affect ranking - only the general guidelines, eg. excessive image content, need be considered.Hope that helps.
If you can't find a site tat offers free photo's, Just give some credits to the official owner of the photo's you're going to use. "Original Image Source: Website"