Krita on Linux as the equivalent to Windows’ Paint
I use such programs primarily to crop and add frames to screenshots. Even for that relatively simple task, Pinta, the first program I tried, is far too unstable (not to say very unstable). Pinta usually works OK for cropping an image, but any drawing operation very quickly makes Pinta freeze (become unresponsive).
On Ubuntu 20.04 MATE with Cinnamon:
sudo add-apt-repository ppa:kritalime/ppa sudo apt-get update sudo apt-get install krita
Create a new image with the same dimensions as the image in the clipboard
E.g., as a result of cutting out a portion of a larger screenshot.
Do menu Edit → Paste into New Image (shortcut Ctrl + Shift + N).
If there is a dialog box, choose “As Web” (shortcut Alt + W).
(In Windows Paint, this was done by starting with an initial image smaller than the one in the clipboard, say 100×100 pixels, and then pasting in the image. The result was a new image with expected dimensions.)
The content in the clipboard could come from a screenshot by Ctrl + PrtSc in Ubuntu MATE 20.04 (Focal Fossa). Note that this copies both screens on a system with two screens. Or Ctrl + Shift + PrtSc for selecting a part of the screen for the screenshot. Or Ctrl + Alt + PrtSc for making a screenshot of the current window.
Change image size (e.g. to make space for a frame)
Get image dimensions: Menu Image → Properties. Close the dialog (e.g., by pressing Enter).
Add two times the expected width for the frame plus a margin (e.g. 2×20 + 10 = 50).
Open a new image (Ctrl + N) and select “Custom Document” (usually the default). Fill in the new dimensions in fields “Width” and “Height”. Press Create (or Alt + C).
Paste in the image and select it. Use the Move tool (on the left, select “Move Tool” (eight row, second column. Or press “T”)) to center the image.
Note: Menu Image → Scale Image to New Size (Ctrl + Alt + I) can not be used because it will also scale the image.
Create a frame for an image
On the left, select “Line Tool” (third row, second column).
On the right, in “Advanced Color Selector”, click on the colour ring. This will not by itself change the colour. In the three rows below the colour ring, click to select a colour. This will change to colour for subsequent line drawing operations.
Set line width: In the toolbar, “Size” (to the right of “Opacity”), click in it to set some size. A drag action will continuously show the line width. It can also be set to an exact size by right clicking (then the line width can be typed in). Finish by pressing Enter. A reasonable size is 20 pixels.
Draw the frames. To lock in the x or y direction (actually, it is 15° increment), hold down the Shift key and move the mouse a little to lock it into place.
Zoom in and out
The menu commands are menu Edit → Canvas → Zoom In / Zoom Out. More interesting are the keyboard shortcuts. They are the same as a web browser (Ctrl + “+” and Ctrl + “-“).
On the left, select “Freehand Brush Tool” (third row, first column).