Man this is weird, a couple of days ago someone tells me he is going to get a pen tablet, so I say cool it is awesome to have a pen tablet you will enjoy it.
2 days later the same guy says " I GOT IT" "Now show me how to do matte paintings" ROFL
Let me explain, the things that you ask are not simple, not easy, and in some cases impossible!

1. How to think of a concept- Impossible to tell you how to think of a concept, a concept comes from each artist creativity, dreams, experiences, fears.  It is (trying not to be rude) a Ludicrous thing to ask, concepts for a piece are what makes an artist, technique is how well they can bring those concepts to life.

2. How to make realistic water or blood. - Google, there are many very generous artists that publish tuts on specific subjects, most of them just give you hints how to do it, others, give you tricks using filters and such. You have to go and learn, read, research, and read some more, then experiment fail and try again.

3. how to spot a good stock for lps.- Read # 1 you use your concept to find the images you need you search the internet sometimes for days, or model and render what you need, nobody can tell you how to spot it you work and get it yourself.

4. where to find good objects/renders for that lp.- Same thing as # 3 plus if you are interested in abstracts, vectors, fractals or 3d rendering.....guess what....You have to learn how to do it again researching reading etc.
There are some great sites where you find good sources for 3d, I use c4d and it is no use linking you to a couple of sites and then finding out you use 3DSMAX ....again Google.

Well hope I was not rude, that was not the intention, the intention is to point you in the right direction, and tell you and many others that ask similar questions, right off the bat that there is no short cut, there is no go around, there is no "Be a great artist in an afternoon" book. Every single piece you see in this group Art Packs, took long hard work to create, and years to learn how to do it.
Cheers hope you do well.