I have some PNGs that I created in Photoshop which have transparent backgrounds. I've saved these as PNGs and when I view the images in my file stystem in my browser, the background is transparent as I expect. However, I uploaded them in Sitecore (using the basic Upload File button in the media library) and saved them, but when I place them on my page they have a white background -- I should be able to see the a background image through them, but instead the background is white.
It seems like the image rendering part is not implemented in correct way. For start with basic you can render the page(consist image) and try to open the image only in new tab(inspect element in browser) and from new open tab try to download the image and see still it is giving you white background or the image is as expected image with transparent background. If image is with transparent background then you need to do change in the rendering logic of your image.