Honestly, your sprites aren't that bad to begin with.
The best advice that I can give is to literally just keep at it. The more you make, the better you get. That's all there is to it, of course if you really want to improve you have to learn from what you create but ultimately it all comes down to just "doing more" and improving personally.
I can say that your bomb animation is nice, but lacks a little... The smoke particles are quite nice but the explosion is what's lacking.
Definitely better work than the average person that's trying to get to grips with pixel art so keep at it and you'll get better with time.
The way that I forced improvement was to force myself every day to make some sprites and tiles, I sectioned off at least 1hr a day to make sprites and I would run a generator to pick what resolution of sprite from all kinds of low res to more default 32x32 resolutions of sprites and I would do the same for colour palettes, I would set them to randomly pic based on old systems and about 6months into things I started adding custom palettes to play with in that selection process, whatever came out I used and I also often had a friend or colleague at the time to randomly pick a 'thing' either an animal, type of character, topic ect and just cracked on with it. I personally prefer limiting myself as much as possible, when you find yourself working with as little as 16x16 and 16-32 colours in your palette you find that it's not nearly as easy as some people think but once you come out of that restriction and you start working with 32x32 you find yourself doing more with less pixels and that allows you to do more with the available canvas. The same goes with the colours and limitations of them, but that's just my process. Everyone works differently.
Just keep creating sprites and you'll get better, post your stuff online wherever you can, back when I did that it was a personal thing and I didn't have a facebook or twitter account, so I didn't do that step, but putting your stuff out thee will help others see your progress, provide some tips and generally share your work.
Based on what you have here the above is the best advice that I feel I can give, you seem to be doing a good job already, with regards to your animated bomb effect, look into adding a little more character to the animation by including some form of anticipation for when the fuse goes down all the way, utilizing a little squash and stretch may work well there, if when the fuse goes down you stretch up, squash down and then it goes boom (or even the other way around), you may get a nicer visual out of the whole animation.
Keep up the great work, your sprites aren't bad!