Pictures size advice

Good evening,

One importante point to define is the size of pictures to go on line.

In GD/design/home there is field to define how uploaded large size pictures should be automaticaly resize by GD in 800×800 or 1024*1024 (for the featured one).

I have made some tests and it looks to me that uploaded images in detail pages (large image then ?) are 800×600 (not 800×800) or 1024*768 (not 1024×1024, and not only the featured one of a place but all of them if option 1024 flaged).

Is that correct or do i miss something?
Why do you speak about large image ?
Do that mean that when one image is uploaded you make varius copies of the pictures (large, medium, little ???).

I have checked from detail page of place or event demo – pictures were 587×387 not 1024×1024 or 800×800 neither 800×600.

What do that mean ?

What do you considere optimum size (:-)) for GD pictures.

If y do upload my place by CVS – and then copy corresponding images in wp_content/wp_upload/2014/11 – what size do you advice me to choose …

Thanks for your help.

Good evening

Yves