Hi there! I did all the graphics of Boot Wizard with Graphics Gale, i think that for small pixel art it is the best one, and its free.
I'm also a lover of 8 bit graphics, and i use the same color palette of the NES for my graphics, you can easily find it on Google.
Some basic recommendations to do such kind of graphics on Construct, is to set the texture filter to point, and the text to aliased, using spritefont might be better though. Also, when working on movement inside the game itself, you should have separate objets for most moving objects, a detector and a sprite containing the graphics with animations, and you just set the sprite position to the same position of the detector, but rounded to a integer, this way you get the pixel perfect movement.
Hope i've helped! Looking foward to see some retro stuff from you!