first of all - get yourself a DSLR. You can start from there.

Don't submit flowers for your initial 3.  Don't submit 2 of the same subject or similar.  Submit at least one with a model.  Get a DSLR.

The photos are not so bad and i do not see any major flaws.
i think you should follow your route.

The subject is fine but your camera is probably not going to cut it, many people now use top of the line equipment (not that you need the best but a DSLR or good mirrorless is really the minimum).  Istock wants variety in the application so three different subjects should be chosen like a portrait, lab still life, and famous landmark all done well will give you a better chance at getting accepted.  My friend was rejected a few times for taking all landmarks, I don't get it since they all would be accepted if they already passed the application but they want variety in your first three.

There is another question too.
How to start promoting my images?
Starting a website or a weblog ?
Or posting on forums (not spamming) and so ??
Or they will find their sales only based on the keywords ?

If you're going to promote your images on your own website or weblog, why would you not get 100% of what the pics earn, rather than give a huge percentage to the agency by linking to your images there?
If you already have a website/blog, a subtle mention wouldn't go amiss, but if you don't have one already, think long and hard before starting one.
(Clue: without a reputation, you'd find it almost impossible to sell through your own agency).
Do you know any forums where people are interested in buying stock images but don't know how to do it?
If so, post there (and tell us about it!). If not, what's the point?

Yes, you pay (i.e. their cut of the revenue) the agencies a lot of money to promote your images. Some advertise/promote better than others. Make sure your keywords are excellent and relevant to your images. Please don't annoy buyers by spamming your images with irrelevant keywords.