Hi A.Borzykh,
Thanks for trying the plugin, I’m going to help you as best as I can.
As stated in the marketplace page :
That will help you to figure out how to create your own brushes. If you encounter a problem with any of those, I will answer you. 
You can also watch the livestream if not done already : https://youtu.be/GDgUe7UGqNo?t=525
Now regarding the problems you pointed :
The “fill preserve alpha” node in your graph turns everything into red. Can you show me the details of the image you are using as stamp, especially its format and compression settings ?
Any texture2D asset will do the job, but the stamp(s) you are using should have the same format as the texture you are editing with iliad (RGB, 8bit, 16bit, etc).
It’s really depending on your computer CPU as everything is heavily multithreaded. Can you tell more more about your config ? Do you mean the size of the stamp or the size of the edited image ?
Iliad should be able to manage alpha channel on almost any image.
It’s right there : https://my.pcloud.com/publink/show?c…7HKiSIVHsqOLmV