The image is of a grey rectangle. As the image opens without errors and doesn't look corrupt so it seems likely that it is a data section not displaying.

Looking at the jpeg spec each marker is prefixed with 0xFF and when there is a 0xFF in the data segment it needs to be followed with 0x00.

https://en.wikipedia.org/wiki/JPEG#Syntax_and_structure

Using a hex editor I searched for all instances of 0xFF and checked that the following byte was either a correct jpeg marker or 0x00.

At offset 0x26f there is a marker that is 0xFC which is not valid.

The previous marker is a 'Start of Scan' (0xDA) and the one following is a piece of data (0x00) so this look like it should be data as well.

Changing the byte 0xFC to 0x00 and saving the file makes the flag visible.

Original writeup (https://github.com/shellfarmer/ctf-writeups/blob/master/seccon2017/jpegfile/solution.md).