Move the cursor with the joystick or arrow keys. 
Place pixel: fire or space 

Left arrow: flip sprite left/right 
Up arrow: flip sprite top/down 

Flood fill: F 

Move sprite: M (I forgot how it works exactly, but you can shift the sprite horizontally and vertically with this) 

Go one sprite up in memory: + 
Go one sprite down in memory: - 

F1/F3/F5/F7: Select drawing color from palette. F1 is 1st color, etc. 
F2/F4/F6/F8: Change color in palette. Order of colors is shown at bottom right. 

CLR: clear sprite 
G: toggle grid lines on/off/flashing(?) 

Undo: R (for restore). Restore sprite is shown at bottom left. 
Copy: B (for Buffer). Buffer content is shown at bottom right. 
Paste: C (for Copy from buffer) 

Load from disk: L 
Save to disk: S 

Basically, the key mappings are all highlighted on the righthand side of the screen. There is nothing more than those. 

I forgot what 'A' (Arrange bitmap) does exactly. Maybe cycling the colors within the palette (and then probably doing the same within the sprite, so only the order of colors changes, but not the sprite appearance). It must be something like that, but I'm not 100% sure. It has been a while.