Here’s a good list of shortcut keys for PAINT.

General Canvas Controls

Hold spacebar + drag with left mouse button Pan image (does not work while typing with the Text tool)
Hold spacebar + drag with right mouse button Scroll image (does not work while typing with the Text tool)
Mouse Wheel up / down Scroll up / down
Shft + Mouse Wheel up / down Scroll left / right
Ctrl + Mouse Wheel Zoom in / out
Esc or Enter Deselect (remove selection)

Image List and Image Navigation

Ctrl + Tab Next tab
Ctrl + Shft + Tab Previous tab
Ctrl + Q Show Image List
Ctrl + # or Alt + #, where # is 1 through 9 for images 1 through 9, or 0 for image 10 Switch to image #, for images 1 through 10
Mouse wheel (only if the mouse cursor is in the image list area) Scroll image list

Toolbar

[ Decrease brush width by 1
Ctrl + [ Decrease brush width by 5
] Increase brush width by 1
Ctrl + ] Increase brush width by 5

Selection Tools (except Magic Wand)

Draw with left mouse button Create new selection, replacing any old selection if there was one
Draw with right mouse button Subtract from the current selection (difference)
Draw with Ctrl + left mouse button Add to the current selection (union)
Draw with Ctrl + right mouse button Invert a given area of the selection (exclusive-or)
Hold Shft while drawing Constrain to square (Rectangle Select), or circle (Ellipse Select)
While drawing with left mouse button, hold right mouse button as well and move mouse Move the new selection while still drawing it
While drawing with right mouse button, hold left mouse button as well and move mouse Move the new selection while still drawing it

Colors Window

X Swap primary and secondary colors
C Switch the active color in the dropdown box

Magic Wand Tool

Left mouse click Create new selection from pixels in the surrounding area that resemble the pixel you clicked on, replacing any old selection if there was one (resemblance is determined by the Tolerance setting)
Right mouse click Subtract from the current selection (difference)
Ctrl + Left mouse click Add to the current selection (union)
Ctrl + Right mouse click Invert a portion of the selection (exclusive-or)
Hold Shft with one of the four cmnds listed above Do a global selection across the entire layer based on the color you click on and the Tolerance setting

Text Tool

Hold Ctrl Temporarily hide the cursor and nub
Tap Ctrl Toggle the nub’s visibility
Esc Finish text and commit it to the layer

Clone Stamp Tool

Ctrl + Left click Set the source point, or origin
Draw with left mouse button Copy pixels (you must have already set the source point, or origin, with Ctrl + left click)

Recolor Tool

Ctrl + Left click Pick up the color under the mouse cursor, and set it as the primary color
Ctrl + Right click Pick up the color under the mouse cursor, and set it as the secondary color
Draw with left mouse button Draw with the primary color, but only over pixels that resemble the secondary color (resemblance is determined by the Tolerance setting)
Draw with right mouse button Draw with the secondary color, but only over pixels that resemble the primary color (resemblance is determined by the Tolerance setting)

Tool shortcuts

S Rectangle Select
S Lasso Select
S Ellipse Select
S Magic Wand
M Move Selected Pixels
M Move Selection
Z Zoom
H Pan (hand)
F Paint Bucket
G Gradient
B Paintbrush
E Eraser
P Pencil
K Color Picker
L Clone Stamp
R Recolor
T Text
O Line / Curve
O Rectangle
O Rounded Rectangle
O Ellipse
O Freeform Shape

File menu

Ctrl + N New
Ctrl + O Open
Ctrl + W, or Ctrl + F4 Close
Ctrl + S Save
Ctrl + Shft + S Save As
Ctrl + P Print
Alt + F4, or Alt + F then X Exit

Edit menu

Ctrl + Z Undo
Ctrl + Y Redo
Ctrl + X, or Shft + Delete Cut
Ctrl + C, or Ctrl + Insert Copy
Ctrl + V, or Shft + Insert Paste
Ctrl + Shft + V Paste in to New Layer
Ctrl + Alt + V Paste in to New Image
Delete Erase Selection
Backspace Fill Selection
Ctrl + I Invert Selection
Ctrl + A Select All
Ctrl + D Deselect

Image menu

Ctrl + Shft + X Crop to Selection
Ctrl + R Resize
Ctrl + Shft + R Canvas Size
Ctrl + G Rotate 90º (clockwise)
Ctrl + H Rotate 90º (counter-clockwise)
Ctrl + J Rotate 180º
Ctrl + Shft + F Flatten

Layers menu

Ctrl + Shft + N Add New Layer
Ctrl + Shft + Delete Delete Layer
Ctrl + Shft + D Duplicate Layer
Ctrl + M Merge Layer Down
Ctrl + Shft + Z Rotate / Zoom
F4 Layer Properties

Adjustments menu

Ctrl + Alt + L Auto-Level
Ctrl + Shft + C Brightness / Contrast
Ctrl + Shft + M Curves
Ctrl + Shft + G Black & White
Ctrl + Shft + U Hue / Saturation
Ctrl + Shft + I Invert Colors
Ctrl + L Levels
Ctrl + Shft + P Sepia

View menu

Ctrl + + Zoom In
Ctrl + - Zoom Out
Ctrl + B Zoom to Window
Ctrl + Shft + B Zoom to Selection
Ctrl + Shft + A Actual Size

Window menu

F5 Tools (toggle)
F6 History (toggle)
F7 Layers (toggle)
F8 Colors (toggle)
Ctrl + Q Show Image List
Ctrl + Tab Next Tab
Ctrl + Shft + Tab Previous Tab

Credit : KeyXL