The Add picture data entry form displays a control with which you can navigate through your local file space and identify a picture to be uploaded. As a follow-up you will see a form with fields for you to nominate the page on which your picture will appear, and to give it a title, and (optionally) some details about it. The details will be displayed beneath the picture if a visitor to your site clicks to see it in its full size. This effectively allows you to add extra pages of content to your site, and include material which would not fit in elsewhere.
The Change picture option allows you to change any of the textual data associated with your picture. You cannot actually "change" a picture — if you no longer want it on your site you must remove it and (possibly) upload another one instead.
The 'Change picture' list also links to an option to Change picture display order. The option shows all the images defined for a given page, allowing editors to give any of them a sequence number to adjust its relative position and override the normal default ordering. It does not operate on pictures of type 'Logo', and the sequence numbers do not affect the display order of 'inline only' pictures.
Remove picture asks you to select a picture by title, then shows it to you while requesting confirmation. To keep your site looking active, it is advisable to "ring the changes" by periodically removing pictures of past events and uploading new ones. At present there is no overall limit to the number of pictures on a site, but you should aim to have an up-to-date selection, with informative names and descriptions.