I was updating my theme with the pretty amazing Twenty Seventeen from WordPress. Attempting to add a site icon it was just… failing. The blue select button in the Media Library would just go grey. When I tried a picture larger than 512×512 pixels I got a step further with the cropping dialogue, but then I’d get…
There was an error cropping your image.
It turns out WordPress needs the php-gd library for image manipulation.
First work out what php version you are running with
On Ubuntu/Debian hosts with php 5.x run
sudo apt-get install php5-gd
On RedHat/CentOS hosts with php 5.x run
sudo yum install php-gd
You may need to install specific packages like php56-gd for php 5.6. For php 7.x you will need to install php7.0-gd
Finally restart your web server
As an aside: I’m regretting my commitment to add an image to every post. What image makes sense with this post? The official logo for GD is 300×230 pixels.