I would use Inkscape

(which you can install from the Ubuntu Software Centre or sudo apt-get install inkscape).

You will need to right click the image, select Open With -> Other Application...
and chose Inkscape from the list. After you have done this the first time, you can just right click -> Open With -> Inkscape.

This will import the image into Inkscape and the page will be sized to fit the image. You can then make your annotations.

To save it, you need to use File -> Export Bitmap, click Browse... and chose your original image.

Flameshot

It comes with some cool annotation features like Freehand drawing, Lines, Arrows, Boxes, Circles, Highlighting, Blur, etc.
It also comes with many on-screen buttons as well lots of customization options.

Its written with QT/C++ and it's very easy to install in Ubuntu(18.04+).

apt install flameshot

You can also try to compile for older versions.

If you want to stay in gimp, you can add boxes and circles/ellipses. Use the rectangle or ellipse selection tool, and when you have the marching ants, on the main menu click Edit/Stroke Selection, then make your line style choice.

There are gimp plug ins to do arrows (that I have not tried), however it took only a few minutes to find a clip art site, copy an arrow image, convert it to a transparent-background gif using these directions, and add it as a new layer to the image. The arrow layer can be moved, rotated, scaled, colored, etc.

You also might want to try GnuPaint or Krita. Not sure if it's exactly what you're looking for, but they might work for you. Both are in the Software Center.

GnuPaint

Krita

You can use firefox or Chrome web browser to do this. On firefox you have to install fireshot addon and on chrome "screen capture"

https://​addons.mozilla.org/en-US/firefox/​addon/​fireshot

Unfortunately fireshot works on Windows only. Here's a list of such addons. Please test them on your own.
https://addons.mozilla.org/en-US/firefox/search/?q=screen+grab&appver=11.0&platform=linux

Screen Capture will do on Chrome

https://chrome.google.com/webstore/detail/cpngackimfmofbokmjmljamhdncknpmg

I recommend the "Awesome Screenshot" extension, You can easily add there text, simple arrows and other shapes.

Google Chrome

https://chrome.google.com/webstore/detail/awesome-screenshot-screen/nlipoenfbbikpbjkfpfillcgkoblgpmj

Firefox

https://addons.mozilla.org/en-US/firefox/addon/awesome-screenshot-capture-

Opera

https://addons.opera.com/en/extensions/details/awesome-screenshot-capture-annotate/

On Ubuntu (or Linux Mint), try Kolourpaint. It's easy to use and has the features you describe.

You can use Pixtick. Web tool, no installation required and free.
http://www.pixtick.com

You can take a look here, u'll find a lot of funny chrome extensions:

https://chrome.google.com/webstore/search-extensions/screenshot?hl=en-US

And this articile talks about some others:

http://www.hongkiat.com/blog/top-web-annotation-and-markup-tools/

Ksnip

Because I run KDE using GTK themed apps like Shutter is not ideal. After doing some research I settled on ksnip as a screenshot tool. It has pretty much everything you asked for (if you need to crop, just do it by taking another screenshot).

Install

You can install ksnip buy downloading the .deb packages here
https://github.com/DamirPorobic/ksnip/releases

or check if it's already in your version of Ubuntu's repository vi apt:

apt install ksnip

I use LibreOffice impress https://www.libreoffice.org/discover/impress/ for annotations.

Just copy paste the image into the slide and add all the shapes and text which will serve as annotations. Then export to any of many available formats, like JPEG, PDF, etc.

Annotating with Impress keeps "annotations" as a separate layer allowing you to edit them at any time. On the other hand, if you annotate with, say, Shutter editing tool, once you saved the image with the annotations, you cannot undo the changes (you'll need to re-annotate the entire image from the initial state or use eraser tool to make corrections to existing annotations).