When uploading photos for your listing (Event or Place), you should try to use high DPI, high resolution images. Images are recommended to be a 2400x1600px, so that they do not appear distorted or pixelated across multiple viewing devices.
It is recommended that you upload at the highest resolution possible, as the website's server will automatically scale the image down and optimize it. However, it can only scale down and not up. Higher the resolution you upload, the better it'll turn out. There is a limit of 16MB (Megabytes) per image, this is to protect the website from abuse. If your image files are larger than 16MB, you may want to try to "Save For Web" using popular image or photo editing software before uploading.
JPEG or PNG images are the officially supported file types. You may attempt to upload a different file type, but it may not be supported.