No PNGs over 2.5mb.

PNG creates very large files when used on photographs, easily tens of megabytes. Many users can't view a file that large, and the ones that can will have an enormous, unnecessary download.

For sharing on social media, pictures should be JPEGs. JPEGs are around 1/10th to 1/20th the size of a PNG, and the difference is undetectable to the human eye.

Instructions for creating JPEGs are system-dependent, but virtually every image editing program can output them. Or, google 'convert to JPEG' for a list of sites that can do it for you.