This article is to help if you've selected your image from your device, but it's not uploading to your Cindr site. If you just want to know how to upload images then there are articles on uploading a background image and uploading a foreground image.
The most likely reason is the file size of your image. Cindr will only let you upload images sized up to 10mb. Images with large file sizes will also slow down the loading time of your website leading to a poor experience for your users. There is plenty of free software available designed to reduce file size, such as Optimizilla and Compressor.
If you're confident that the images are under 10mb, and it's a background image you're trying to upload then it's possible that you can't see the image through the background colour. To check this adjust the transparency by using the slider between the