Wow. That's a lot of feedback, thank you. ... and no, I'm not too busy... lol
At the moment, my intention is to keep the sprites 'square'. I'm still trying to figure out how to code for 8x8, 16x16 and 32x32. As I was planning on simple square sprites, rectangular or other shapes, wasn't even thought of... Most of the larger editors can define the size of the image when "New" is selected. Unfortunately, my current skill level, cannot reproduce that function. I'm not saying that it cannot be done... I'm saying that it cannot be done by "me"... *sigh*
Oh boy. Sprite-sheets... I have always wanted to be able to create and or separate sprites from a sprite-sheet. At the moment I use Gimp to extract sprites. I have no idea as to how to do that with QB64 (or any other basic)...
How many colours? Oh man, that is such a loaded question. If you are creating 'old school' games that use 8x8 or 16x16 then use a simple collection of colours. Have a close look at Mario. Usually between 3 to 6 colours were enough for Mario. 32x32 plus sprites obviously can contain more colours. My editor has only 117 colours. But not all of them are used. Selecting a palette of colours (just like an artist) is the most important thing to consider when using colour. It's not how "many" it's more about how the colours are "used". For example, Let's say that you use 20 colours to make level one of a game, be consistent, use the same colours for the other levels. Too much variation can be a distraction.... That being said... Standard RGB will number 16,777,216 colours...
Is MSPaint a good tool for sprite creation? I am a Linux user and have not used mspaint for more than 15 years. There are 'many' choices out there for graphics editing... From the simple (like mspaint) all the way up to Photoshop and beyond. All of those editors are good tools. The question might be better: Which editor is good for "you"? My suggestion: Start small. Not sprite size but complexity of the editor. If you are comfortable using mspaint, then stick with it. Piskel is a free online editor. There are many to choose from. Here is a 10 minute youtube that will cover free and non-free editors. I hope this helps.
These are only a few but it should give you a good place to start...
Saving and loading images to and from my program is still an ongoing issue. There are people, smarter than I, who are assisting with the problem...
I hope I haven't confused you... I tend to "waffle" on like that if I haven't had my coffee... NO! It's almost 10am and no coffee!!