Cannot upload certain images (icons)

I’m trying to upload some social media icons that I downloaded, and the upload does work but once the upload is complete I cannot see a thumbnail and if I go to the image permalink there is no image there. At first it was in png format, then I exported the image to jpg and tried again, but same problem. I then tried uploading other images and that works fine. It only seems to be the case with the icons, and I also tried uploading two different icons downloaded from two different sources. I even tried resizing the icons, but nothing works. I then tried it with a different site, and same problem. Any ideas why that’s happening? Thanks.