GIMP, or the GNU Image Manipulation Project is the standard Linux program for manipulating images. And it is great. I have to say I use it more than Krita although that maybe because I am more used to it. If you want to resize, crop, or layer your images (or for that matter any number of other edits) then GIMP is a great program to install.
To install GIMP on KDE neon open Konsole and type,
sudo apt install gimp
Then press enter, authenticate with your password, and answer any questions asked of you. After installation GIMP can be found in the Application Menu under Applications/Graphics or by selecting the menu and typing its name.
Tips and tricks
There are a couple of things I do after installing GIMP. The first is to switch it to single window mode. This is a matter of personal taste but having three windows wandering around instead of one annoys me.
To do so, go to the Windows option in the top menu and select Single-Window Mode. This will bind all your windows together in a way that I find easier to work with.
Another thing I do, not because I need to but because I like playing with the extra options, is to install the Data Extras package for GIMP. To install the Data Extras package on KDE neon open Konsole and type,
sudo apt install gimp-data-extras
Then press enter, authenticate with your password, and answer any questions asked of you. After installation you won't notice too many differences until you look under the Tools or Filters menus where you will have a much greater range of options.