Is there any Megapixel limit for upload photos?
When I hit "Process to publish" one file (7000x7000pix) Im getting blank page with title "Process Images" only.

There is often a memory limit with your hosting company if the images never show up in the All Images tab.  This link talks about raising the memory limit.

http://cascoly-images.com/pix/how-do-i-solve-problems-caused-by-low-memory/

I discovered today I was still using the old image processor not the preferred ImageMagick processor (If you go to the Image Processing tab it will say using ImageMagick under the Process Images Header)

http://www.microstockgroup.com/19525/19525/msg321175/#msg321175
Describes how to change to ImageMagick if needed.

Is there any Megapixel limit for upload photos?
When I hit "Process to publish" one file (7000x7000pix) I'm getting blank page with title "Process Images" only.

Try to process smaller one, the look under "Process Images'. Do you see "Using ImageMagick"?
If there is GD, it needs huge memory to process files. In your case 7000x7000x3 bytes are used to load file, about 150MB. So memory limit for php on your server should be set to 256MB, to process it (memory for other purposes is needed too). You can try to change this limit (how, it depends, sometimes it is setting in admin panel, sometimes in php.ini in main folder).
But GD is bad solution, it always generates less or more visible horizontal banding in resized images. Much better solution is to use ImageMagick. If you don't have it, ask support, how to enable it on your server. Is is much less memory consuming, 128MB should be enough, even maybe 64MB.

Done, two seconds on cPanel

does this help
http://wiki.dreamhost.com/ImageMagick_and_imagick_php_module_on_shared_hosting

Another problem appeared.
I already have ImageMagick but I can only process about 12-18 photos in one batch.
If I check more than that Im getting:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.


I have:
memory_limit   512M
max_input_time   -1
max_file_uploads   3000
max_execution_time   10000
upload_max_filesize   1500M